It sounds like you selected the wrong download from the AVG site (unless you really wanted the trial version of the commercial software).
AVG Free Edition is not time-limited and is as good (or better) than anything else available. Uninstall your current version of AVG and then install the one you can get by using the 'Download Now' link in the yellow box (or the link immediately underneath it, in the orangish-brown box):
http://free.grisoft.com/doc/5390/lng/us/tpl/v5
Using that version of AVG, you'll still have the same level of anti-virus protection as before but without any time restrictions.
Chris