Let's all post our little secrets (some not so secret) little programs that help people keep their PCs clean, efficient, and safe!
AVG (free): http://free.grisoft.com/
Runs in your browser for a thorough, accurate, and current scan. It takes a long time but picks up many threats that a lot of others miss
Malware / Spyware / Spam removal:
SpyBot: Search and Destroy (free) http://www.download.com/3000-2144-10122137.html
(also available at many other sites)
Ad-Aware (free): http://www.lavasoftusa.com/
Use both of these programs for the best results, one will pick up some threats that the other will not.
Pop-up stopper (free): http://www.panicware.com/
Zone Alarm used to be free, I'm sure you can find some links around that still hae the free version.