Running a macro when the active excel window changes to another application window

Hi
I want an excel macro to run when the activewindow changes from an excel window to a window of a different application. So, for example, if I were to have excel open with its window showing and word is running minimized, if I clicked on the word (bottom) taskbar button to bring up the word window my excel macro would run.
I have tried an application windowde_activate event in a class module but this doesn't seem to work. Am I doing something wrong??
Thank you
George