Hi Zeb Yes, I too run multiple antivirus, but only two. Avast and Adaware. Both paid pro versions. They seem to co-exist happily and one catches some things the other doesn't. Any more would be overkill I think. A full rescan of all drives by both progs at least once a month just to be sure. I have tried other programs, but some conflict with each other and with firewalls.
Note that while Adaware free does a good job, you get what you pay for. The full version gives pretty much total antivirus coverage - not just ads/spam. When it comes to security, don't be stingy. It could cost you everything, trying to save a buck.