You should see more information on that screen, such as the Current registration expiry date and the number of days remaining.
Try expanding that window and see if that reveals the additional information.
If not:
Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 or later - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
Reboot after the repair.