NVIDIA never signed on with Microsoft and so none of NVIDIA's drivers are "digitally signed". When WinXp does its update it apparently downloads an updated graphics driver for the NVIDIA cards. The problem is that they are not NVIDIA drivers .. they are Microsoft's closest approximation.
I don't know if this is your problem, but I have seen something similar correcetd on two other machines so far running XP with NVIDIA cards.
Solution: go to NVIDIA
http://www.nvidia.com/content/drivers/drivers.asp
and download the latest graphics driver for your card
Hope this helps.