Certainly Sergey's choice is better -- use SYSMETRIC with no trouble.

If you still want to use the GetWindowRect:

DECLARE INTEGER GetDesktopWindow IN user32
DECLARE SHORT GetWindowRect IN user32;
	INTEGER hwnd, STRING @lpRect
LOCAL hDesktop, cRect
hDesktop = GetDesktopWindow()
cRect = Repli(Chr(0), 16)
= GetWindowRect(hDesktop, @cRect)
? dword2num(SUBSTR(cRect, 1,4))
? dword2num(SUBSTR(cRect, 5,4))
? dword2num(SUBSTR(cRect, 9,4))
? dword2num(SUBSTR(cRect, 13,4))

FUNCTION dword2num(lcBuffer)
RETURN Asc(SUBSTR(lcBuffer, 1,1)) + ;
	BitLShift(Asc(SUBSTR(lcBuffer, 2,1)),  8) +;
	BitLShift(Asc(SUBSTR(lcBuffer, 3,1)), 16) +;
	BitLShift(Asc(SUBSTR(lcBuffer, 4,1)), 24)