My pc is too slow, what should I do?

Mozilla firefox freezes for a few seconds, youtube videos and wmp/vlc videos won't play normally, sound is ok but video freezes too. In general my pc is ridiculously slow. I formatted my disk, then installed drivers and only a few programs such as vlc, msn, google earth, antivirus, office, programs that shouldn't do any harm. I also downloaded microsoft updates for XP. But here I am with the same problem about 2 or 3 days after the format. I do not enter in dangerous sites so I can't understand what the hell is going wrong!! Any ideas??