Avast! - Great detection & low memory use

Avira AntiVir - No email scanner; update nag screen

AVG Free - Popular, but uses more resources

Emsisoft Emergency Kit - Make sure you update before running

Anti-Virus

A good, full-time anti-virus program is the core of your computer security and helps keep the nasties from getting on your computer in the first place. Make sure you only use only ONE of these.

PC Security

Manual AV Scanner

Not a substitute for your primary defense, but useful for exorcising some stubborn malware.

Bitdefender - At last a free option for one of the best anti virus programs