With Windows 98 and avast, Thunderbird must be configurated manually. Change the setting of the email accounts into Thunderbird: Menu Tools > Account settings this way:

Pop3 (Inbound):

Server name: 127.0.0.1
User name: YourUserName#YourPopServerName
Port: 110

Smtp (Outbound):
Server name: 127.0.0.1
User name: YourUserName
(or, in some systems, user name is: YourUserName#YourSmtpServerName)
Port: 25

Note, probably you should run Mail Protection Wizard to have ashMaiSv.exe automatically running at Windows startup.

Do you use any spam killer?