Why don’t you try to delay the starting of the update with NetRun. This application detects the connection state and lauch programs automatically. You can set the programs or just drag & drop them into the main window.
Put there the C:\Arquivos de programas\Avast\aswUpdSv.exe and set it for running some seconds after the connection be done. Probably in Windows 98 you will have to delete some Startup item (in Windows XP is a service called avast! iAVS4 Control Service that must be stoped).
I know it’s not the final solution but untill there… Anyway, you will love NetRun… 