#uselib "user32.dll"
#func global GetCursorPos "GetCursorPos" var
#func global ScreenToClient "ScreenToClient" int, var

dim point, 2

repeat
GetCursorPos point
ScreenToClient hwnd, point

redraw 0
color 192,192,192
boxf
color 0,0,0
pos 0,0
mes "(" + point.0 + ", " + point.1 + ")"
redraw 1
await 16
loop

win32APIを使ってみた。これでおk?