I work for a free PC Security utility. (www.threatfire.com)
and I can tell you from my personal experience, they're all about the same. For pay. Not for pay. It doesn't matter. Some clean things up better than others. Some detect better than others. Some have lots of fake hits, some have very little fake hits. Some are incredibly alarmist, and some are very understated. But, in the end, they all block the majority of the baddies, and in the end, they all have the potential to save your behind from danger.
I, however, have come to the conclusion that security software can be more or less avoided, if you know what you're doing. Don't go to dirty links, do click on suspicious content, keep the privileges of your account to a minimum (LUA with that annoying but ever-so-useful UAC), update windows often and with everything, and if you need to do something that even feels like it COULD be fishy (opening a habitually-infected friends' USB drive on your PC) use a virtual machine. (Virtual Machine 2008 and Linux are both free, walk you through the process, and more or less transparent to use.) Hell, if you're ultra-paranoid, Virtualize EVERYTHING, and leave only your most-trusted apps on your host for heavy-lifting.
AV's are not protection, they're damage control. Behavior guys (Like us, PrevX, and HIPS like Comodo and so forth) are approaching the more "proactive" solutions, but in the end, things like Security Essentials, Norton, and Avast are exactly what they say they are: Virus removal tools that might help you stop it from hurting your system farther once it's on the box.
IMO, just being intelligent about security and your sites (only go to trusted sources, don't do shady things on a production machine, keep everything separate, don't open things you don't know) and keep yourself very well informed before venturing into the nasty open world, and everything will be peachy. Why get sick and need an immune system when you can use test mules and knowledge to avoid being sick in the first place?