Free Antivirus for Windows - Open source GPL virus scanner
also use
http://www.safer-networking.org/index2.html which is Spybot. Spybot has a memory resident program called teatimer. It monitor registry changes and prompts you if you want to allow the change. If you are not installing or updating something, it is probably a good idea to deny the change.
Also, to save headaches. Always, let me repeat,
always, create a regular user account to browse with. If the user you are using has no authority to install or modifiy anything on the system except own user stuff, the viruses and trojans and other nasties cannot go memory resident except in user space or install themselves in system areas.
It may be a bit more inconvenient to have to switch users to update, install, or remove, but it is far easier than an OS reinstall because of some nasty that the anti-malware software doesn't catch or clean.