Frankly, I think AV packages are near useless if you are reasonably careful. They bog your system down. They don't defeat the latest viruses/worms as the authors of those things test against the latest AV packages. And you really aren't at risk for this things if you don't randomly download and install things from questionable sources or email.
If you are looking to protect a PC that you have kids working on, then assuming you are using XP, setup user accounts for them that don't have privileges to install products and configure a lot of things. Then become religious about not letting the kids use the system without being logged into one of those restricted accounts. If it's a problem with an SO, then do the same to them
