If you are going to use a free Antivirus try Microsoft Security Essentials. It is actually a very good one. The scans could be a little faster, but it is light on resources and works perfectly with Windows. I also use the free version of Malwarebytes Antimalware and scan once a week with it after updating it of course. It doesn't run in the background so it is just a scanner and remover, but it will get rid of things that your regular antivirus won't. It is great at getting rid of those rogue viruses that are spreading. If you do a lot of downloading I highly suggest to have an on demand scanner like Malwarebytes or Spyware Terminator installed.
