I don't know why you would go from Vista to XP without contacting support. If I purchased Vista and had that problem I would have contacted them. Were you using a pirated version or something? I can't figure out why you would buy Vista and then move to XP.

Edit: As a side note the 8800 has some serious heat issues using it's stock fan and heatsink. Do you have any utility installed to show it's temp? I bet thats where your problem is.