I've got a couple of machines at home that I protect using OneCare. It's convenient, reliable and, unlike some other products I could mention, isn't a resource hog.
I was quite excited this evening to see that the team has released a pretty major update that improves the protection and optimization for both individual machines as well as a home network.
The update will roll out to existing users over the next few weeks so keep an eye out for it!
For more information, check out OneCare. If you're interested in a more corporate solution the bow-tie wearing chap in the office across from me pointed out that Forefront is its more enterprise focused cousin.