Funny, I was just reading a blog article today by security expert Bruce Schneier where he claims "... you won't notice any performance degradation at all" in reference to running an anti-virus application in the background [source
]. At first, I thought he made a simple typo. After rereading it, I figured he was just being facetious. After completing the article, I now realize that he actually believes this statement to be the truth!
Sometimes, it is difficult to judge just how accurate Bruce is when it comes to computer systems security.
Anyway, the anti-virus game is a downhill losing battle. Your best bet is dumping it in favor of some more reliable (though less user-friendly) methods of security, such as default-deny ("white-listing") and running your user accounts with the principle of least privilege. This is why OS X and Linux are often called "more secure" than Windows; because they use these ideas (with varying levels of success) in their default configurations. Unlike Windows! (And god damn it, don't disable UAC if you're stubborn enough to stick with Windows! It's really the only thing you need to keep your dumb ass safe on the interwebs. Even though it is ridiculously flawed and trains lusers to blindly click the "allow" button ... it's a lot better than nothing!)