To those people having problem with their computers because they don't have any Anti Virus or having problem because their trial/activation code has expired. Here I made a research for the Top 10 Free Anti Virus so that you can download and you will not be worrying anymore with your anti virus that expires.
1.AVG Free - http://free.grisoft.com
2.ClamWin - http://www.clamwin.com/content/view/18/46/
3.PC Tools Anti-Virus 3.1 Free Edition - http://www.pctools.com/free-antivirus/
4.A-squared Free - http://www.emsisoft.com/en/software/free/
5.AVAST 4 Home Edition - http://www.avast.com/eng/avast_4_home.html
6.Avira AntiVir PersonalEdition Classic - http://www.free-av.com/
7.BitDefender 8 Free Edition - http://tinyurl.com/34nzqo
8.Google Pack - http://pack.google.com/intl/en/pack_ins…&gl=us
9.TrendMicro HouseCall - http://housecall.trendmicro.com/
10.VCatch Basic Version 5 - http://www.vcatch.com/download.html
hope it can help you.