Get the paid version of Malwarebytes' Anti-malware. In my experience (professionally repairing PCs for going on six years), malwarebytes is the best program available. Technically it's for spyware, malware, and rootkits, but it catches trojans and viruses better than most actual antiviruses. It's very light on resources, so you could pair it with a free antivirus like AVG or Avira.
http://www.malwarebytes.org/mbam.php
I have never used antivirus on my PCs. (Mainly use MAC though.) But I do have two PCs and have never used antivirus and have never had a virus ever.
Nod32.
More secure than free ones, doesn't run 8 different processes.
Anything else is an exercise in nubbery.
Just delete system 32 and you're good to go.