Just to clarify - there are two versions of Avast Antivirus. One is free, but the other one (called 'Pro') is not free.
As Broni says, you can re-register the free version every 12 months without paying anything. They make you register it evry year basically so that they always have your current email address.
Main difference between the free and 'Pro' version is the free version cannot block malicious scripts when your web-browsing, which does mean you're more vulnerable to those types of threats. But for a free antivirus I think Avast still takes some beating and I recommend it over the other free ones.
There is a less well known one from China however called "Rising Antivirus", which seems to be fully loaded with all the features of a commercial one, if you can put up with it's quirky interface: http://www.freerav.com/
Edited by phillipcorcoran, 05 July 2010 - 10:14 AM.