Changing from fullscreen to window and vv

[Jimmy Valavanis]

System: Windows XP, P4 3GHz, nVidia 4400
Compiler/IDE: Delphi 6 Personal
API: DirectX 7 (DirectDraw)

I'm working in an game using DirectDraw(not Direct 3D) and I have a little problem changing from fullscreen to window mode and vise-versa:

If I'm in window mode and change to fullscreen the screen resolution changes correctly but nothing is drawn (I see the empty window background) Here is my code:

var
g_pDD: IDirectDraw7 = nil; // DirectDraw object
g_pDDSPrimary: IDirectDrawSurface7 = nil;// DirectDraw primary surface
g_pDDScreen: IDirectDrawSurface7 = nil; // DirectDraw surface


// ................
......
procedure SetupDrirectDraw;
begin
.....
end;

procedure I_ChangeFullScreen;
var
hres: HRESULT;

begin
if fullscreen then
begin
hres := g_pDD.SetCooperativeLevel(hMainWnd, DDSCL_NORMAL);
if hres <> DD_OK then
begin
Error;
exit;
end;
end
else
begin
hres := g_pDD.SetCooperativeLevel(hMainWnd, DDSCL_EXCLUSIVE or DDSCL_FULLSCREEN);
if hres <> DD_OK then
begin
Error(true);
exit;
end;
end;
hres := g_pDD.SetDisplayMode(SCREENWIDTH, SCREENHEIGHT, 32, 0, 0);
if hres <> DD_OK then
Error(fullscreen);

fullscreen := not fullscreen;
end;

The I_ChangeFullScreen procedure tries to change from fullscreen to window mode and from window to fullscreen. The error function never called (i.e. DirectDraw does not return an error) but when I call my drawing function nothing is drawn:

procedure I_FinishUpdate;
var
srcrect: TRect;
begin

srcrect.Left := 0;
srcrect.Top := 0;
srcrect.Right := SCREENWIDTH;
srcrect.Bottom := SCREENHEIGHT;

if g_pDDSPrimary.BltFast(0, 0, g_pDDScreen, srcrect, DDBLTFAST_DONOTWAIT or DDBLTFAST_NOCOLORKEY) = DDERR_SURFACELOST then
g_pDDSPrimary.Restore;

end;

Starting the program I call the SetupDrirectDraw procedure that setup DirectDraw. After that everything works fine. But when the I_ChangeFullScreen() is called (using ALT+ENTER keys) the computer's monitor change resolution but nothing is drawn.

The SCREENWIDTH and SCREENHEIGHT are 640 and 480.

Any ideas?