Das machst Du dann so (ohne das ganze Winstation-Gelumpe):
Delphi-Quellcode:
HDesktop := OpenDesktop ('Winlogon', 0, False, DESKTOP_ALL);
if (HDesktop <> 0) then
begin
if SetThreadDesktop (HDesktop) then
begin
DoScreenshot('Winlogon');
CloseDesktop (HDesktop);
end;
end;