The best Anti-Virus software according to me is Kaspersky 7 and if you buy it cost rs.750/= and you will get one year validity with three user can use it and it will be good and it does not allows any virus to affect your computer system
Free anti-virus..AVG is good.
If you can pay, i would recommend NOD32.I have used it and it is simply great.Won't allow any virus to work.
By the way, if you need anti-virus for temporary use, then you can download a trial version of NOD32.It is a 30 day trial and all the features will be available.I once got a virus and i removed it using the trial version.
Usually, i use Linux . so, no problem regarding viruses.But,When i use Win 7 sometimes, i use COMODO firewall+antivirus. Its free.I didn't have single virus and trojan problem since in last 5 years. its really good.
