You need to uninstall any pre-release version of the Microsoft Office 2007 system .
it seems that if you have a version of the 2007 Microsoft Office Add-in
or Microsoft Save as PDF or XPS installed, this may be the culprit. In
both cases, by performing an uninstall on this add-in via the Control
Panel, and then installing Expression Web, everything works okay. You
can even re-install the Office 2007 add-in when you're done