Windows XP Home locks up returning to the welcome screen

I have a system running Windows XP Home, when
you manually swith users or it automatically
switches back to the welcome screen, the system
locks up about 25% of the time. The mouse still
works, but no selections on the welcome screen
will highlight or activate. It happens with or
without hibernation turned on. No system errors.
My guess is the nvidia driver because it usually
happens after someone has used the system for
gaming and I have had lots of other issues with
the nvidia driver. Anyone have any suggestions?
Yes switching to an ATI card instead is on my
list of possibilities. Thanks — fred.