Free antivirus download?

Hi Ladies,
Recently my colleague borrowed my pen drive for her personal use and i got it back with a virus attached :-( which affected my laptop at home!
It was a regsvr.exe virus which usually gets transaferred thro pen drives and mainly it attacks the registry!
i somehow managed to clean it up!
To avoid this in future, can anyone suggest any good antivirus open source software? I tried the trial version of Spyware doctor but now it comes with a cost.
if anyone is aware of any free antivirus s/w pls point me to that!
Will be very helpful!

Thanks,
Jaya