Das Programm könnt ihr selber testen und von http://www.virtualuna.de/giveme/Project1.exe runterladen. Nur 33kb groß. Bräuchtet aber WinXP wegen der GDI+.DLL.

Code:

__fastcall TForm1::TForm1(TComponent* Owner)
  : TForm(Owner)
{
  Gdiplus::GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::FormDestroy(TObject *Sender)
{
  delete myPen;
  delete Bmp;
  Gdiplus::GdiplusShutdown(gdiplusToken);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::PaintBox1MouseMove(TObject *Sender,
      TShiftState Shift, int X, int Y)
{
  Gdiplus::Graphics gfx(Bmp);

  if(mdown)
  {
    //Bmp->SetPixel(X, Y, Color::Blue);
    gfx.DrawLine(myPen, X1, Y1, X, Y);
    PaintBox1->Repaint();
  }

  X1 = X;
  Y1 = Y;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::PaintBox1MouseDown(TObject *Sender,
      TMouseButton Button, TShiftState Shift, int X, int Y)
{
  mdown = true;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::PaintBox1MouseUp(TObject *Sender,
      TMouseButton Button, TShiftState Shift, int X, int Y)
{
  mdown = false;  
}
//---------------------------------------------------------------------------
void __fastcall TForm1::FormCreate(TObject *Sender)
{
  myPen = new Gdiplus::Pen(Color::Blue, 1);
  Bmp = new Gdiplus::Bitmap(640,480, PixelFormat32bppARGB);
  Gdiplus::Graphics gfx(Bmp);
  gfx.Clear(Color::White);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::PaintBox1Paint(TObject *Sender)
{
  Gdiplus::Graphics gfx(PaintBox1->Canvas->Handle);
  gfx.DrawImage(Bmp, 0,0);
}
//---------------------------------------------------------------------------

Vielen dank schonmal