A second endorsement of SysInternals. I found these before MS bought them out, when I was trying to revive a blown out computer, suspected of housing a rootkit. They are a bit technical for the average user, but some of what they do is invisible and extremely valuable.

I also use a purchased AVG (Grisoft) on my main machine, plus Spybot and AdAware, and things are just fine, technically. I purchased AVG w/firewall, but I couldn't get a satisfactory configuration (beyond my tech knowledge and patience), so I went back to free ZoneAlarm.

M