Download and save the free version http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe and the removal tool http://www.avast.com/uninstall-utility

Uninstall your current copy then reboot the system to safemode ( tap f8 while starting the system ) once in safemode run the removal tool for all versions of avast ever installed and once completed reboot back to normal mode and install the new free version.