Lets see.. Pc works fine without the card, but screws up with the card.. I would suppose it may be the card. Why not try a different one? (come to think of it, who is sitecomm? All I can find online is they seem to be a european company)
I just wonder why, in your own words, you say you know the problem is not the pc or xp, so why do you keep reformatting the hard disk/reinstall xp etc, and refuse to use one of the many dozens of cards people use all the time with no problems, but keep reinstalling a card that causes problems?!?!
I suggest trying a different card...