Ron, I have had no 'real' problems with XP and Nvidia cards.
I did have a problem with my monitor not being properly supported by XP.
I am impressed with the performance of ATI Radeon cards. If you truly wish to try a different card, I would start there.
I have had XP problems with several different modem cards.
While the cards worked fine with the same motherboard and W98, they just refused to work properly with XP.
This does appear to be a card/motherboard conflict, as the same modem works fine in a different brand motherboard.
Before you give up, check the following:
BIOS updates.
Correct CPU speed settings.
Windows Updates. (some of the compatability updates correct hardware issues)
Latest Nvidia driver.
Have you checked out the shutdown trouble-shooting guide posted at www.aumha.org
While they might not have your specific issue listed, it has helped me get on the right track on a couple of installations.
Chas