POINT Pts;
RECT WndPos;
GetCursorPos(&Pts);
GetWindowRgnBox(hwnd, &WndPos);

while(TRUE){
	Sleep(1);
	if(Pts.x => WndPos.left && Pts.x =<WndPos.right && Pts.x <= WndPos.top && Pts.x => WndPos.bottom){
		if(Pts.y => WndPos.left && Pts.y =<WndPos.right && Pts.y <= WndPos.top && Pts.y => WndPos.bottom){
			//Send the message or whatever
		}
	}
}