Hello All,
Beginning yesterday, the definitions update feature, automatic and manual, reports that my license key has become “expired or invalid.”
Attempts to reinsert the valid license key into its configuration field fails.
All other functions of the program continue to work.
What’s up?
I use Windows 98 SE.
Cheers, John
Welcome…
Can you check if your system date/time are ok (specially the year)?
If so, can you try to copy&paste the registration number again?
A few days ago, while updating, a message did arrive from Avast to check the time and date as it was in error and was interfering with a certificate.
I did resync my time and date against the atomic clock.
Changing to a new registration number did not help; and
the update problem persisted until only moments ago when I replaced the servers.def with one I found at Avast. 
Cheers,
John
You can always register avast! again and receive a new registration number… 
If you want, you can try the build 4.5.529?
More informations here: http://forum.avast.com/index.php?board=2;action=display;threadid=9007