If you are running WinXP you should try Microsoft's Anti-spyware.
You can get it here.
Windows AntiSpywareIf you don't have a legal version of Winxp installed you won't be able to use it. If this happens to be the case for you can try this instead.
SpybotAVG Anti-Virus is good. I had that for years. I recently updated to avast!. One of the two big reasons I changed is because it had boot time scan. AVG use to have that but they got rid of that feature. I missed it. You can try it out, but I found the controls a bit hard to understand at times. It's a good thing they have skins for the program Or I would have gone back to AVG I'm sure.
avast! Link.Oh... all programs listed are free.