AVG - Grisoft have a free edition of their anti-virus software available, updates availably frequently, non-intrusive, ad/spyware free, easy to setup and very stable.
Freeware version available, registration required (also free).
AntiVir - Personal edition, support available via a forum (mostly in German).
Free for personal use.
BitDefender - Anti-virus and security software, free Anti-Virus versions available.