The bug
After installing the latest trial version of Expression Web, every time Outlook 2003 is opened a dialogue box pops up saying “Please wait Windows Installer is configuring Expression Web” or something to that effect.  It would run through the configuration setup and ask to restart Windows. Even after restarting Windows the dialogue box would reappear.

The solution
The solution (for us) was quite simple. In Outlook 2003, select Help > Detect and Repair from the main menu. You will need to insert your installation disc for Microsoft Office. Immediately after running Detect and Repair, the problem seems to have gone away.

Other people having this problem have reportedly reinstalled both Outlook and Expression Web to no avail.