I use Avast! personally, and also Spybot - Search and Destroy. They've both worked fine for me so far. :3 They're both free, too.
You'll need to register for a free home license on Avast!, and you'll need to get a new license every so often (every year I think), but that's easy, because you've already registered, you can easily get a new license. The info's on their site. This may seem like a bit of a hassle, but Avast! can also run in the background, protecting your computer. Spybot also has a similar feature, which allows you to disallow registry edits by programs. This may be a bit too advanced for some (and it can mess up things if you install a program but don't allow the registry changes it needs) so it's a feature you have to be careful with.