Dim OldWindowProc As Long
Dim hwnd As Long
Dim lngCurPos As POINTAPI
Dim R As Range

Sub SubClass()

'Subclass The ONLY loaded workbook Window
hwnd = Application.hwnd
hwnd = FindWindowEx(hwnd, 0, "XLDESK", vbNullString)
hwnd = FindWindowEx(hwnd, 0, "EXCEL7", vbNullString)

OldWindowProc = SetWindowLong(hwnd, GWL_WNDPROC, AddressOf
NewWindowProc)

End Sub

After the Sub named SubClass is run, doesn't the variable OldWindowProc (as
well as all of the other module-level variables) go out of scope? This would
mean that VBA would throw the value away (clear it?), meaning it will no
longer be available for later calls when the mouse moves over the window and
needs to use it again.
--
Regards,
Bill