Another vote for AVG free. As Spiros said some versions definitely slowed down my PC but the latest version I installed 2 days ago seems to be much better.
Microsoft Security Essentials. It's free, easy to install & to use; its unobtrusive and it's good. I stopped wasting money on the bloated Norton & McAfee type solutions some while ago.
Totally agree, I've seen the full version of AVG fail far too often let alone the free version, I use MSE on my laptop and have had no problems and neither have any of the machines I've removed AVG from.
Microsoft Security Essentials definitely. I've used it to clean machines that have been infected even with a full version of Norton or McAfee supposedly protecting it.
Highly recommended, and can now be used in Small Businesses up to 10 PC's I think.