MSE missed a dll virus on my computer once. I didn't notice for a week until my buddy's MSE picked it up from the file I sent him.
What amuses me is that most people believe that computer security is only protecting your WoW account and having to reformat. There are things you do on your computer that are much more threatening: online banking, credit card purchases and filling out your SSN are just a few things that people can get their hands on. Identity theft is much worse than getting your WoW account hacked.