For Windows based machines, I used to recommend AVG. Its free, does a decent job, and isn’t too horribly invasive like some software I could mention (*cough* McAfee and Norton *cough*).
…and then Microsoft came out with their own “Security Essentials” free anti-virus. I was dubious at first. If its Microsoft I expected it to be behind the times, bloated, and generally a “hanger on” in the Anti-virus game.
I was wrong. Security Essentials works, is lightweight, stays out of the way, has developed a good track record, and has the single best feature I’ve seen in any antivirus software in years: it automatically updates its definitions before it scans. Wow, brilliant, and nobody else does it.
So if you are using Windows, do yourself a favor and get Security Essentials (from here: www.microsoft.com/security_essentials) instead of any of the others.