This may sound kind of strange, but with my ATI video card, even though it was a 8x card in an 8x slot, it started at 4x, and I had to change it in windows by going to display properties. I don't remember the name of the tab I had to click, and I'm at work instead of home.
I know, ATI instead of NVidia, but it's something to check. Also, I've got Win 2000 instead of XP.
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