Can you send and receive emails without attachments?
You have said it doesn't really matter what size the attachments are, you still have the problem.
What type of file are you attaching, .exe, .doc, .zip, .jpg, etc. ?
How did you set-up your email programs to work alongside avast's email scanner?
With win98 you need to manually set-up or use the Mail Protection Wizard - check out this thread -
User's FAQ - Mail Protection Wizard thread, it will give you a lot of useful advice.