But, besides the automatic configuration, can you manually update?
If not, you have a proxy setting trouble or a firewall block.
The following programs should be allowed to connect through your firewall:
C:\Program Files\Alwil Software\Avast4\ashWebSv.exe (avast! Web Scanner)
C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe (avast! e-Mail Scanner Service)
C:\Program Files\Alwil Software\Avast4\Setup\avast.setup (avast! Update executable)