Aquí tienes un ejemplo:
Código:
procedure TForm1.FormCreate(Sender: TObject);
var
f: HRGN;
begin
Form1.Borderstyle := bsNone;
f := CreateRoundRectRgn(
0, // x-coordinate of the region's upper-left corner
0, // y-coordinate of the region's upper-left corner
clientwidth, // x-coordinate of the region's lower-right corner
clientheight, // y-coordinate of the region's lower-right corner
40, // height of ellipse for rounded corners
40); // width of ellipse for rounded corners
SetWindowRgn(Handle,f,True);
end
Un Saludo.