I need a professional!!!

Ok guys, my computer has a problem. It has been running slowly lately. This all started about 2 weeks ago. Back then my computer could run half-life 2 itunes and counter strike all at the same time without any lag. However, the lag that is occuring with my cmputer now, is only noticeable when im using programs. For example, games that use to take seconds to load are now taking minutes. In addition, all my games are suffering tremendous screen lag. My computer has 1gb ram and 3.2 ghz. It is currently running on wndows xp. Is it a virus? i have tried scanning, but nothing seems to be able to fix my lag. Please help me!!