I can't fault Avast as a running scanner. It pretty much picks up anything (no virus scanner is going to be able to detect every single virus). I also use Malwarebytes Antimalware, which I set off every month to pick up stuff the virus scanner may have missed. If you want that one to run in the background, you have to pay for it, but the free version does an excellent job when you manually run a scan.

If you know what you are doing and looking for, then RogueKiller is another excellent program. If you don't know what you're looking for, then don't use it, as this program can remove programs that are essential to the running of your computer.