It is important to keep whatever anti-virus software you use up to date. Having the software but not keeping its virus definitions current is useless.
A few of the best that you may obtain at no cost are...
Avast! Antivirus
AVG Free Anti-Virus
Microsoft Security Essentials
Avast! and AVG have been repeatedly evaluated positively for years. Microsoft Security Essentials is fairly new and may not be quite the caliber as the other two but it is pretty darn good.
Stay away from Norton and McAfee.
Norton used to be very very good until they were absorbed by Symantec. It was all downhill from there.