You will need to download the update directly from the website yourself. The file can be obtained from this link.
I have installations of Avast! 4.8 Home Edition on two different Win98 systems, installed months apart. On one of them the virus database last updated on December 29, on the other it was January 2. When I click to manually update the database, they hang as if they can't connect to the server.
Are you using Avast! 4.8, and are you still getting virus database updates?
The definitions are updated daily. I have the link bookmarked for easy access. It is unknown how much longer Avast will continue to provide 4.8 updates. Expect support to cease at any moment.