I installed the VIA Hyperion driver, I tried to install the card
with WinXP SP1, SP2 and even SP3. I have Ghost backups of various
installations of WinXP, so it wasn't too time-consuming to try
them out.

I strongly suspect an incompatability with the VIA chipset drivers and the
various nVidia drivers you have tried. Which version of the chipset drivers
are you using, the 5.1's?