Since its XP, you tried a repair? Also, how long has it been going on? When it first started, did you try a system restore back to before the issue began?

Could be hardware related also, do you have any extra ram to swap and test with? If not, you can try pulling 1 stick out at a time to see if that makes any difference.

What does performance monitor show when its going slow? How much ram free, how much processor free, etc?