Download both AdAware Personal SE and SpyBot Search & Destroy. What one doesn't catch, the other usually does...

Onto AntiVirus - at the very least get AVG Free, aswell as AVAST if you want.

Run all of the above. I'd also run the Housecall from TrendMicro (visit thier site) to be safe....

After that, set the computer to defrag (START > ALL PROGRAMS > SYSTEM TOOLS)