Здравствуйте, Krinkels!
Программа написана в MS Visual C++ 2008 Express Edition (полагаю, что проект будет открываться и в Visual C++ 2010 Beta). Для создания графического интерфейса пользователя использованы Windows Forms (проект Windows Forms Application). Для отображения на главной форме (TMainForm) числовых значений байт, килобайт и т.д. использованы числовые поля ввода - объекты класса NumericUpDown: BytesSpinBox, KiloBytesSpinBox, MegaBytesSpinBox и GigaBytesSpinBox. Для контроля изменения содержимого этих полей предусмотрены переменные



которые принимают значение true, если пользователь изменял значение числового поля (вводил в него значение) - см. реализации функций System::Void KiloBytesSpinBox_ValueChanged(System::Object^ sender, System::EventArgs^ e) и т.п. класса TMainForm.
Реализация логики работы программы представлена в функции нажатия на кнопку (System::Void CalculateButton_Click(System::Object^ sender, System::EventArgs^ e)). Расчёт основан на том, что 1 килобайт равен 1024 байт, 1 мегабайт - 1024 килобайт и т.д.

Внешний вид программы представлен на рисунке ниже:



Проект программы прикреплён к ответу.

P.S. А вобще рекомендую сменить среду разработки. Ту же задачу средствами G++ / Qt решить было бы гораздо удобнее, да и приложение получилось бы кроссплатформенным.

Приложение:
// Файл TMainForm.h

#pragma once


namespace Bytes {

using namespace System;
using namespace System::ComponentModel;
using namespace System::Collections;
using namespace System::Windows::Forms;
using namespace System::Data;
using namespace System::Drawing;

bool bBytes = false;
bool bKilobytes = false;
bool bMegabytes = false;
bool bGigabytes = false;

public ref class TMainForm : public System::Windows::Forms::Form
{
public:
TMainForm(void)
{
InitializeComponent();
//
//TODO: Add the constructor code here
//
}

protected:
/// <summary>
/// Clean up any resources being used.
/// </summary>
~TMainForm()
{
if (components)
{
delete components;
}
}
private: System::Windows::Forms::Label^ BytesLabel;
private: System::Windows::Forms::Label^ KiloBytesLabel;
private: System::Windows::Forms::Label^ MegaBytesLabel;
private: System::Windows::Forms::Label^ GigaBytesLabel;
private: System::Windows::Forms::NumericUpDown^ BytesSpinBox;
private: System::Windows::Forms::NumericUpDown^ KiloBytesSpinBox;
private: System::Windows::Forms::NumericUpDown^ MegaBytesSpinBox;
private: System::Windows::Forms::NumericUpDown^ GigaBytesSpinBox;
private: System::Windows::Forms::Button^ CalculateButton;


protected:
private:

System::ComponentModel::Container ^components;

#pragma region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
void InitializeComponent(void)
{
this->BytesLabel = (gcnew System::Windows::Forms::Label());
this->KiloBytesLabel = (gcnew System::Windows::Forms::Label());
this->MegaBytesLabel = (gcnew System::Windows::Forms::Label());
this->GigaBytesLabel = (gcnew System::Windows::Forms::Label());
this->BytesSpinBox = (gcnew System::Windows::Forms::NumericUpDown());
this->KiloBytesSpinBox = (gcnew System::Windows::Forms::NumericUpDown());
this->MegaBytesSpinBox = (gcnew System::Windows::Forms::NumericUpDown());
this->GigaBytesSpinBox = (gcnew System::Windows::Forms::NumericUpDown());
this->CalculateButton = (gcnew System::Windows::Forms::Button());
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->BytesSpinBox))->BeginInit();
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->KiloBytesSpinBox))->BeginInit();
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->MegaBytesSpinBox))->BeginInit();
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->GigaBytesSpinBox))->BeginInit();
this->SuspendLayout();
//
// BytesLabel
//
this->BytesLabel->AutoSize = true;
this->BytesLabel->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif", 8.25F, System::Drawing::FontStyle::Bold, System::Drawing::GraphicsUnit::Point,
static_cast<System::Byte>(204)));
this->BytesLabel->Location = System::Drawing::Point(19, 18);
this->BytesLabel->Name = L"BytesLabel";
this->BytesLabel->Size = System::Drawing::Size(52, 13);
this->BytesLabel->TabIndex = 0;
this->BytesLabel->Text = L"Байты: ";
//
// KiloBytesLabel
//
this->KiloBytesLabel->AutoSize = true;
this->KiloBytesLabel->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif", 8.25F, System::Drawing::FontStyle::Bold, System::Drawing::GraphicsUnit::Point,
static_cast<System::Byte>(204)));
this->KiloBytesLabel->Location = System::Drawing::Point(19, 48);
this->KiloBytesLabel->Name = L"KiloBytesLabel";
this->KiloBytesLabel->Size = System::Drawing::Size(80, 13);
this->KiloBytesLabel->TabIndex = 1;
this->KiloBytesLabel->Text = L"Килобайты: ";
//
// MegaBytesLabel
//
this->MegaBytesLabel->AutoSize = true;
this->MegaBytesLabel->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif", 8.25F, System::Drawing::FontStyle::Bold, System::Drawing::GraphicsUnit::Point,
static_cast<System::Byte>(204)));
this->MegaBytesLabel->Location = System::Drawing::Point(19, 78);
this->MegaBytesLabel->Name = L"MegaBytesLabel";
this->MegaBytesLabel->Size = System::Drawing::Size(81, 13);
this->MegaBytesLabel->TabIndex = 2;
this->MegaBytesLabel->Text = L"Мегабайты: ";
//
// GigaBytesLabel
//
this->GigaBytesLabel->AutoSize = true;
this->GigaBytesLabel->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif", 8.25F, System::Drawing::FontStyle::Bold, System::Drawing::GraphicsUnit::Point,
static_cast<System::Byte>(204)));
this->GigaBytesLabel->Location = System::Drawing::Point(19, 109);
this->GigaBytesLabel->Name = L"GigaBytesLabel";
this->GigaBytesLabel->Size = System::Drawing::Size(78, 13);
this->GigaBytesLabel->TabIndex = 3;
this->GigaBytesLabel->Text = L"Гигабайты: ";
//
// BytesSpinBox
//
this->BytesSpinBox->Location = System::Drawing::Point(150, 18);
this->BytesSpinBox->Maximum = System::Decimal(gcnew cli::array< System::Int32 >(4) {-1530494976, 232830, 0, 0});
this->BytesSpinBox->Name = L"BytesSpinBox";
this->BytesSpinBox->Size = System::Drawing::Size(120, 20);
this->BytesSpinBox->TabIndex = 4;
this->BytesSpinBox->ValueChanged += gcnew System::EventHandler(this, &TMainForm::BytesSpinBox_ValueChanged);
//
// KiloBytesSpinBox
//
this->KiloBytesSpinBox->DecimalPlaces = 2;
this->KiloBytesSpinBox->Location = System::Drawing::Point(150, 48);
this->KiloBytesSpinBox->Maximum = System::Decimal(gcnew cli::array< System::Int32 >(4) {1215752192, 23, 0, 0});
this->KiloBytesSpinBox->Name = L"KiloBytesSpinBox";
this->KiloBytesSpinBox->Size = System::Drawing::Size(120, 20);
this->KiloBytesSpinBox->TabIndex = 5;
this->KiloBytesSpinBox->ValueChanged += gcnew System::EventHandler(this, &TMainForm::KiloBytesSpinBox_ValueChanged);
//
// MegaBytesSpinBox
//
this->MegaBytesSpinBox->DecimalPlaces = 3;
this->MegaBytesSpinBox->Location = System::Drawing::Point(150, 78);
this->MegaBytesSpinBox->Maximum = System::Decimal(gcnew cli::array< System::Int32 >(4) {100000000, 0, 0, 0});
this->MegaBytesSpinBox->Name = L"MegaBytesSpinBox";
this->MegaBytesSpinBox->Size = System::Drawing::Size(120, 20);
this->MegaBytesSpinBox->TabIndex = 6;
this->MegaBytesSpinBox->ValueChanged += gcnew System::EventHandler(this, &TMainForm::MegaBytesSpinBox_ValueChanged);
//
// GigaBytesSpinBox
//
this->GigaBytesSpinBox->DecimalPlaces = 6;
this->GigaBytesSpinBox->Location = System::Drawing::Point(150, 109);
this->GigaBytesSpinBox->Maximum = System::Decimal(gcnew cli::array< System::Int32 >(4) {1000, 0, 0, 0});
this->GigaBytesSpinBox->Name = L"GigaBytesSpinBox";
this->GigaBytesSpinBox->Size = System::Drawing::Size(120, 20);
this->GigaBytesSpinBox->TabIndex = 7;
this->GigaBytesSpinBox->ValueChanged += gcnew System::EventHandler(this, &TMainForm::GigaBytesSpinBox_ValueChanged);
//
// CalculateButton
//
this->CalculateButton->Location = System::Drawing::Point(195, 168);
this->CalculateButton->Name = L"CalculateButton";
this->CalculateButton->Size = System::Drawing::Size(75, 23);
this->CalculateButton->TabIndex = 8;
this->CalculateButton->Text = L"Рассчитать";
this->CalculateButton->UseVisualStyleBackColor = true;
this->CalculateButton->Click += gcnew System::EventHandler(this, &TMainForm::CalculateButton_Click);
//
// TMainForm
//
this->AutoScaleDimensions = System::Drawing::SizeF(6, 13);
this->AutoScaleMode = System::Windows::Forms::AutoScaleMode::Font;
this->ClientSize = System::Drawing::Size(292, 216);
this->Controls->Add(this->CalculateButton);
this->Controls->Add(this->GigaBytesSpinBox);
this->Controls->Add(this->MegaBytesSpinBox);
this->Controls->Add(this->KiloBytesSpinBox);
this->Controls->Add(this->BytesSpinBox);
this->Controls->Add(this->GigaBytesLabel);
this->Controls->Add(this->MegaBytesLabel);
this->Controls->Add(this->KiloBytesLabel);
this->Controls->Add(this->BytesLabel);
this->Name = L"TMainForm";
this->Text = L"Пересчёт";
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->BytesSpinBox))->EndInit();
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->KiloBytesSpinBox))->EndInit();
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->MegaBytesSpinBox))->EndInit();
(cli::safe_cast<System::ComponentModel::ISupportInitialize^ >(this->GigaBytesSpinBox))->EndInit();
this->ResumeLayout(false);
this->PerformLayout();

}
#pragma endregion
private: System::Void BytesSpinBox_ValueChanged(System::Object^ sender, System::EventArgs^ e)
{
bBytes = true;
bKilobytes = false;
bMegabytes = false;
bGigabytes = false;
}
private: System::Void KiloBytesSpinBox_ValueChanged(System::Object^ sender, System::EventArgs^ e)
{
bBytes = false;
bKilobytes = true;
bMegabytes = false;
bGigabytes = false;
}
private: System::Void MegaBytesSpinBox_ValueChanged(System::Object^ sender, System::EventArgs^ e)
{
bBytes = false;
bKilobytes = false;
bMegabytes = true;
bGigabytes = false;
}
private: System::Void GigaBytesSpinBox_ValueChanged(System::Object^ sender, System::EventArgs^ e)
{
bBytes = false;
bKilobytes = false;
bMegabytes = false;
bGigabytes = true;
}
private: System::Void CalculateButton_Click(System::Object^ sender, System::EventArgs^ e)
{
double dBytes = Convert::ToDouble(BytesSpinBox->Value);
double dKiloBytes = Convert::ToDouble(KiloBytesSpinBox->Value);
double dMegaBytes = Convert::ToDouble(MegaBytesSpinBox->Value);
double dGigaBytes = Convert::ToDouble(GigaBytesSpinBox->Value);

if(bBytes)
{
dKiloBytes = dBytes / 1024;
dMegaBytes = dKiloBytes / 1024;
dGigaBytes = dMegaBytes / 1024;
}
else if(bKilobytes)
{
dBytes = dKiloBytes * 1024;
dMegaBytes = dKiloBytes / 1024;
dGigaBytes = dMegaBytes / 1024;
}
else if(bMegabytes)
{
dKiloBytes = dMegaBytes * 1024;
dBytes = dKiloBytes * 1024;
dGigaBytes = dMegaBytes / 1024;
}
else if(bGigabytes)
{
dMegaBytes = dGigaBytes * 1024;
dKiloBytes = dMegaBytes * 1024;
dBytes = dKiloBytes * 1024;
}

BytesSpinBox->Value = Convert::ToDecimal(dBytes);
KiloBytesSpinBox->Value = Convert::ToDecimal(dKiloBytes);
MegaBytesSpinBox->Value = Convert::ToDecimal(dMegaBytes);
GigaBytesSpinBox->Value = Convert::ToDecimal(dGigaBytes);
}
};
}