Daemon,
It appears that you may have a corrupted installation file.
At this point, I would do 2 things:
Go to the Avast site and download Avast Uninstall Utility and run it. This will clear your system of any leftover files from previous installations.
Secondly, Download a fresh copy of Avast Pro demo.
Use the fresh copy for a new install.
Before trying the install, shut down all unecessary Windows programs except Explorer and Systray.
(Use Ctl-Alt-Del to bring up the Dialog box)
Next, look in the Avast Help file for instructions on configuring Outlook Express. You must change the server address in OE to 127.0.0.1 (local computer) so Avast can scan email, and the username will also be modified using a # symbol. All this will be in the help file.
See if this helps.