hi.....
it's my answer but it's totally you choice which you install :
1) avast! (preferred because free,resident scanner,fast update,boot time scan).you can download home edition free of cost from '
http://www.avast.com'
2) avg -- it is more used because it is infective in finding new viruses. you can get on many of sites as '
http://www.download.com'
3) McAfee -- this used by the old user (not updated with current trend) generally it is not more effective as above.
4) Quick-heal -- is very good anti virus compare to avg and McAfee but it is not available for free u can download from it's site '
www.Quickheal.co.in/downloads.asp'
5) kaspersky -- it is also good anti-virus software but it gives problem with some system,it is also not free software u have to buy first and u can download it from '
www.kaspersky.com'
6) avast!(professional) -- is very good anti-virus includes 99% of all security issue, u can buy it's licence by going on the site '
http://www.avast.com'
finally, the decision is yours