Defragging in XP

A couple of weeks ago, my computer came up with a false security alert, I knew right away it was a virus. I found w32.spybot.worm and after several attempts at removing it, I gave up. I repartitioned my hard drive and reformatted, and reinstalled. My problem is my computer has slowed down since the virus. Now it will not defrag, it analyzes and when finished I choose defrag and it just sits there. I have PC Tools spyware doctor running and just downloaded malwarebytes, it is scanning and says there are no threats. Can someone help?