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 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.
You didn't specify exactly what you trued:
Try double clicking the license.avastlic file that you saved to your hard disk.
If the above doesn't work a clean reinstall is your next option, more on that if required.
Is it avast Pro or avast Internet Security that you have installed ?