You miss one crucial bit of information, what version of avast are you using ?

If avast 4.8 the pop-up is more to do with obsolescence than the license has expired. Avast are trying to get those who can (not on win9x or winME) to update to the latest version of avast 6.0.1289 as 4.8 is effectively on life support and avast are getting ready to throw the switch.

If it is not avast 4.8 - Try a repair of avast:
Win7 - Control Panel, Programs & Features, uninstall a program, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow. You may need to reboot after the repair.