if it is sort of slow "all of a sudden" I would guess a virus...like a trojan.....go to
www.antivirus.com do the free scan (patience, it takes a while)
also, as stated, could be totally due to spyware....I just worked on a dudes comp..the comp was built right, cuz I built it,lol..or at least I formatted and reloaded xp....at the time I didnt know about spyware too much.....and this dude goes to lots of, uhh, "adult" sites....anyway, he called saying his comp was running terrible etc etc....so I got there and ran a virus check but he was clean on that.....so I installed adaware 6 and he had like 150 or 300 (I cant remember) instances of spyware......so I deleted everything that adaware found and the comp was instantly fixed.
www.lavasoftusa.com
after you install it...MAKE SURE..you do the update for it to get the latest defintions...when I did that guys comp his firewall was blocking me from updating the adaware....so I just ran it unupdated.....it found like a couple hundred or whatever....THEN I realized about the firewall blocking it from updating...so I bypassed the firewall and got the latest update for adaware...reran it..then it found like 50 more.....so without the update I would have missed like 50 spywares and it would only have been "sort fixed"
His comp was hanging, wouldnt open My Computer, taking like 4 minutes to boot..I mean all kindsssssssssssssssss of issues.....100% caused by spyware.
JP
If the virus scan and adaware dont fix it..you can always re-install windows "over the top"..in other words by choosing an option NOT to format and with XP choosing just to "repair" the current windows install.
Also...tell what Operating System you have....it makes it easier to help