Comment Re:Simplest answer (Score 1) 835
I heartily second the parent. Wipe it and reinstall. If you've got everything backed up, that should be the quickest option. (Versus spending a weekend or so digging and digging to find the problem.)
Along those lines, this fall, both my and my wife's machines (XP, full updates, behind a hardware firewall,Avast on one and AVG on the other) started off slowing down and then devolved to really wacky virus behavior over the course of a week. Had to wipe both to come back to normal.
Post-wipe, if it's slow, then I'd echo the disk sentiments elsewhere in the comments.