Hovering mouse pointer over window activates it

I’ve got a user who just started having a strange problem on his computer. Whenever he moves his mouse pointer off the currently active window in Windows 2000 Pro, the window that the pointer hovers over becomes the active window and the other window disappears (becomes inactive). I read about the ActiveWindowTracking registry key causing this behavior, but it is set to 0 (off). I’ve scanned the computer with up-to-date antivirus software and spyware removal programs, but haven’t found anything on the computer. I’m about ready to just format the computer and reinstall the o/s, but wanted to check to see if anyone has heard of this happening.