I have AVG(paid version)for virus, and another vote for malwarebytes
I have a few PC's at home. We run either Avast or AVG, along with Adware or Spybot. We have had a few trojans that have caused havoc on the family PC and one of my sons. In these persistent cases where the software finds, but can't get rid of the problem I just do a google search on the file. This will normally take me to thread on a spyware info forum where it is being discussed. They usually have some way of tracking the files and instructions on what to delete.
I have a iMac as my desk top. I didn't worry about anti virus for a while, but then I installed Microsoft office for mac and thought that an anti virus might be worth having. I use PC Tools Anti Virus, a free Mac specific software. So far it has never found anything, which is amazing because just about every time you run Adware or Spybot you come up with a few on the PC's.
Cheers
Col