I have a listbox that covers an entire form. (Don't ask - there is a good reason)

Anyway my problem is that when the person moves the mouse off the listbox (and thus the form) I want some code to run....sooooo I put it in the MouseLeave() method of the listbox. My problem is that unless the user is moving the mouse reeeeeeallly reaaaaaaly slow - the event doesnt run.
Any suggestions on a fix for this??

Thanks!