I was little confused how to convert my AVG Trial version in AVG free version, Because uninstalling AVG and reinstalling process takes time, So i would like to suggest you a easy way for how to convert AVG trial into AVG free version without Uninstalling-
1. In windows XP Go to start>Control Panel > Add Or Remove Programmes and Select AVG Antivirus Click Uninstall.
In Windows 7 Start> Control Panel>Programs>Programs and features Select AVG and clik uninstall (as shown below in screenshot)
2.Now AVG uninstall asks you to convert your trial version to free version see screenshot-
Select Convert AVG 2012 Free version.
Now it will ask you to restart your computer to complete conversion.
NOTE:-It converts it to AVG free Version If you want full version you need to Buy a License from AVG.