I suggest you use a free antivirus!
my favorite choice is Unthreat antivirus.. you can pair it with Comodo firewall to get maximum protection!
no need to spend $$ on a good antivirus!
Well if you have the money to buy a antivirus from market, then go with Norton360 or McAfee 2013, But for running this best antivirus. your PC must be very good, Because it’s to good to protect the PC from virus coming from other PC or internet and even from hacking. [ It’s my own opinion ]
Now if you want to go with freeware antivirus, then go with avira or avast + comodo internet security firewall and spybot search and destroy.
For downloading freeware software, I prefer and refer you to go to the given link. FileHippo.com - Download Free Software