function SetFullscreenMode:Boolean;
var DeviceMode : TDevMode;
begin
with DeviceMode do begin
dmSize:=SizeOf(DeviceMode);
dmBitsPerPel:=32;
dmPelsWidth:=1024; // Тут ставить разрешение
dmPelsHeight:=768;
dmFields:=DM_BITSPERPEL or DM_PELSWIDTH or DM_PELSHEIGHT;
result:=False;
if ChangeDisplaySettings(DeviceMode,CDS_TEST or CDS_FULLSCREEN) <> DISP_CHANGE_SUCCESSFUL
then Exit;
Result:=ChangeDisplaySettings(DeviceMode,CDS_FULLS CREEN) = DISP_CHANGE_SUCCESSFUL;
end;
end;

procedure RestoreDefaultMode; // для возврата дефолтового разрешения
var
T : TDevMode absolute 0;
begin
ChangeDisplaySettings(T,CDS_FULLSCREEN);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
if setFullScreenMode then
begin
sleep(7000);
RestoreDefaultMode;
end;
end;