Wow,
Seems like you've covered your bases, If you hadn't re-installed etc (3 Xs). there is a free program that with reset the XP MCE i've used it successfully many times to avoid digging into the registry to manually reset for it and possibly screwing up the registry....the three installs I assume where clean formats etc they would effectively been the long way of doing what the program does. (WHY THE HELL XP MCE doesn't have that built in IDK. And with SP3 it kills there Power toy for MCE... been over a year and they still refuse to do the simple code update).
Wow, I've got no suggestions BUT I'm very interested in if you ever found a solution to the problem. Or did you just end up using the card in the old system? Or did the card go bad?
I had an issue with a new TV Wonder 650, my very first card/installation which made it all the more frustrating where the Tuner was not detected... after I pulled it out and reseated it things went much better but I swear the first time it was in there straight and secured etc so not sure what was different but without doing the reseating I couldn't boot into XP and could barely boot into Vista.