The best antivirus programs in the world will only catch 70% of spyware, so, that means a miss is inevitable with very program out there. It is going to happen, sooner or later.
The reason we sell avast!; I have never had a virus spread through an avast! protected network in the 9 years I have sold and supported avast!. I cannot make that statement about any other product, though I am limited to McAfee, Symantec, Norton, AVG, and MSE. This is the primary goal of network protection, is too keep the network clean. The avast! network shield is superior, in my opinion.