Lately I've been using Microsoft Security Essentials (MSSE) and Malwarebytes' Anti-Malware (MBAM) to fight malicious software (malware) including virus, Trojan Horse, spyware and other varieties.
Both programs are free and are very effective. I used to use AVG and Ad-Aware, but it seems that lately those programs haven't kept up with the latest threats as well as MSSE and MBAM. In fact, I've seen more than one computer get a very nasty infection while AVG was installed and running.
So, if you want the best protection from malware (for your Windows PC) that no money can buy, get MBAM and MSSE. They are both available from Ninite.com.