san,
This problem more than likely is not with Avast.
I believe that we need to configure your Mail Options settings to allow attachments to be opened.
Outlook Express has a feature which monitors email and will not allow access to what it considers "potentially dangerous....virus infected"
Here's what you can try:
Open up OE and then Tools/Options/Security
Under Virus Protection (the top area), under the box next to "Do not allow attachments..........."
Close down OE, and restart computer.
This should take care of it. If not, let me know.
Good luck,
Techie101