I agree with Spybot and HijackThis. To add to the list Spyware Blaster helps block spyware from coming on your computer in the first place.
I'm a big user of Hijack This. I'm normally remove items fairly hapzardly, but you probably shouldn't:
BHO
Browser Help Objects for IE
you really shouldn't need these, but you may want these. My gut says DELETE THEM.
If it the label indicates Norton Anti-virus, Spybot, or Spyware Blaster, you can keep these.
Run
Anything that says RUN make note of what looks suspicious. Then click on Start, Run, and type in MSCONFIG. Click on Startup. Uncheck the suspicious object(s). Click Apply. Then restart when it asks. Make sure your anti-virus and other needed startup items work. If not run MSCONFIG again and recheck the item, click apply, and restart; no harm, no foul.
BE CAREFUL.
HOSTS
Hosts can be used by hijackers to keep you from running online anti-virus or spyware programs. Gut says REMOVE THEM, but if you're using Spyware Blaster or Spy Bot, it adds bad sites instead.
REMOVE ANYTHING before running Anti-spyware to start fresh.
Toolbars
You really don't need these, REMOVE THEM. Some toolbars such as Google, MSN, or Yahoo aren't dangerous, but be careful.
I remove viruses and spyware from my customer's computers and this has been been successful for me.