- reinstall the mouse driver
- doing a sfc /scannow from the command prompt (keep your XP CD at hand)
Aren't those file you deleted still in the recycle bin? Then simply restore them.
A few weeks ago my mouse's middle scroll wheel stopped working. It can't scroll up or down on a page, but I can still click with it and it scrolls in video games or other applications. I'm using a Dell Bluetooth mouse, but I tried switching to other mouses and they don't work either. It might have happened when I thought I had a virus and deleted a few files that were marked for deletion by Hijackthis, but I don't really remember.