In order to scan your email (with win98), avast needs to change your ISP settings so it can intercept your email and scan it before it arrives in your inbox.
Run the Mail Protection Wizard this should set up your account's server settings to do this. Your firewall needs to allow ashMaiSv.exe internet access.
Check your avast help file about the Resident Scanners - Internet Mail provider and also this thread - visit the
User's FAQ thread, it will give you a lot of useful advice.