The thing is a lot of people don't install a virus scanner until
after they suspect they have a virus, or else they don't keep the definitions updated. The first thing a lot of viruses will do is intercept any API calls to the virus scanner, effectively hiding themselves from it's scan. That is, if they get into the system in the first place, which can only be done on a system without virus scanning or with outdated definitions. I really like Norton, and it works very well, but I have also switched to AVG for monetary reasons.
I think the AVG interface is sort of crappy, but it works, and is free, so that's pretty much all that matters. Just remember, if you let your definitions in AVG get out of date you'll still be at risk as well.