Outlook/Exchange Provider Problem

I recently installed Outlook 2007 onto an XP Media Center notebook which has had avast already installed, and I’m having problems with the Outlook/Exchange provider and add-in. The following message appears when starting Outlook:

The Add-in "avast! 4" (C:\Program Files\Alwil Software\Avast4\ashOutXt.dll) cannot be loaded and has been disabled by Outlook. Please contact the
Add-in manufacturer for an update. If no update is available, please uninstall the Add-in.

So, of course, I tried all the solutions found in the forums including deleting the ‘extend.dat’ file, uninstalling/reinstalling avast and Outlook, using CCleaner, and using the Norton removal tool (I had Norton systemworks 03 running without NAV).

The other problem is that clicking on the Customize button for the Outlook/Exchange provider results in the following warning:

Access is denied.
The operation could not be completed.

This seems strange because I am using an administrator account. Please note we were able to set up another PC running XP Home, Outlook, and avast without any problems.

Some system specs: HP Pavilion dv6119, XP Media Center SP3, AMD Turion X2 TL-50, 1GB RAM, MS Outlook 2007 (updated to latest 12.0.6320.5000), avast! Pro 4.8.1229.

Any help would be appreciated!