I may have found an answer to this puzzling issue. Check out this link....
http://support.microsoft.com/default...b;en-us;305581
It in a nutshell is telling us that if a systems uptime is allowed to accumulate then that can effect how certain animations are processed and displayed.
That will go a long way to explaining why after I reinstall my video drivers things are back to normal for a while. Reinstalling the drivers necessitates a system reboot which resets the uptime back to counting upwards from zero again.
Now, how does one reset the uptime counter to zero without have to reboot the whole system. That may be impossible or I just can't find a method to do it anywhere.
If you are running windows xp you can type 'systeminfo' in the console window and your current uptime will be displayed.
Maybe I am barking up the wrong tree but........