My wife's computer started doing that and it turned out she had a bunch of virus' on her comp, so try checking for that if you haven't done so.
Lately I noticed my computer would restart once in a while when I wasn't around, and now it's so frequent, if it even gets to boot into xp pro before restarting, it will resart soon after. Sometimes when it restarts I get no video. All of my virus/adware programs are up to date and used regularly so I don't think it's a virus probelm. My first thought was that it was a video card problem, but I don't have an extra one to switch it with to see.
I run xp pro sp2
asus a7n8x dexlux
pny georce 5200ultra
Any ideas would be greatly appriciated.