Microsoft Security Essentials. It's actually good.
What Nige said.
I disagree, my computer ended up buggered while running this, I've gone to the darkside and now running norton, 3 licences for £29 and it doesn't seem to slow the system down like it used to! Manages to pick up on loads of things, even when using stumble it stops exploits running from the browser.
Seriously worth a look at the demo if you're not convinced, especially if you have 3 computers in need of protection