[angvelem]

А разве так нельзя?

Код:

  ...
  DC := GetDC(0);  

  bmp.Width := Screen.Width - 400;
  bmp.Height := Screen.Height - 400;
  StretchBlt(bmp.Canvas.Handle, 0, 0, bmp.Width, bmp.Height, DC, 0, 0, Screen.Width, Screen.Height, SRCCOPY);  
  ...