AVG free version is good as it scans your emails too. The free one doesn't detect spyware/adware though so run it with Spybot S&D or AdAware.
Make sure you download the FREE version, NOT the 30 day free trial.
Also, if you have Norton installed, make sure you completely remove it before downloading AVG otherwise it causes chaos with your computer!!
I don't know how true it is but they say that the purchasable AVG is targetted more at businesses, so they're happy to offer it as a free download to home users - the only downside being that you don't get the tech support (although they do have a users forum which I used to find answered questions much quicker lol!)