AVGFree and Avast! are good antivirus programs and they're free

Be vigilant! Scan all files that arrive in your computer.
Also, you shouldn't run more than one antivirus at the same time (i.e. 2 antivirus programs at startup). Could create some nasty conflicts.
And get anti-spyware programs like adaware. Spyware could be a greater nuisance than viruses