[Stony]

The SP demo doesn't contain compressed textures, so setting the textures all the way to full will cause some performance problems at higher rez. At least that's what I've read. The retail game won't have this problem however, so I don't think it's a WinXP problem or your system specs either.

Hardware drivers could possibly contibute to your issues. What settings do you have enabled for video card itself? Are your drivers current for all of your hardware? There's a lot of things to look for, but to make things simple it's mostly because the demo doesn't contain compressed textures.