I use Spybot Search & Destroy, but if you use that one you have to be prepared to close all of your browsers when it's time to scan. It pops up (!) and asks if you want to start a scan.

Bill D has a good list of security software that I've relied on over the years. An excellent one to install is WinPatrol. The idea is that it guards your computer's registry. If any new software tries to sneak in and install itself, it needs to add a line in the registry in order to be run in the system. If WinPatrol detects any change to the registry it pops up and asks you if you want to approve the change. If you weren't doing anything like installing a new program, then the answer is usually "no."