have you tried this :
"Q: My licence key has expired, what should I do?
A: First, check your license key validity. You can do it this way:
Right-click on the blue a-ball icon in system tray.
Select "About avast!..." from the menu.
In the window that appears, unroll the "Registration key" item.
Check "Updates expiration" date and compare it to the current date.
Now you can see if your license (registration) key has really expired. If the updates expiration date is older than the current date, you have to obtain and enter a new license key. It means:
If you are using avast! Professional, you need to buy a new license. Contact your distributor or use our online secure store.
If you are using avast! Home, you can simply register again and get a new key by filling in the registration form.
Once you get the new license key, enter it into avast!.
If the license key seems to be valid (the updates expiration is in future), you should check your system date. It is probably not set correctly. In that case, set the system date correctly and enter your license key again. "