i would never pay for an anti-virus most of the free ones are actually superior, id say the two best are avg 9.0 or microsoft security essentials, id give microsoft the edge as there making an anti-virus to protect a system they designed so there bound to know the weak spots better than anyone else, and i would use malwarebytes to scan your machine for malicious software, all of these are free programs, and you can get them at www.freewarefiles.com or www.cnetdownloads.com
They can't even create an OS without security holes in it,