Although I love our Expression Web product, especially for its CSS editing capabilities, I wouldn't consider myself an expert. I've used it to create some simple web pages and to edit the look-and-feel of my blog. But, over the past couple of weeks, two people have contacted me about problems installing Expression Web on their Windows Vista machines with Office 2007.
If you take a look at the ReadMe, you'll see that: "You need to uninstall any pre-release version of the Microsoft Office 2007 system such as the Beta before installing Expression Web." Those instructions seem pretty clear. However, the two people who contacted me had "clean" systems that had been built from the ground up with the RTM versions of both Windows Vista and Office 2007. There were clearly no prior beta versions hanging around.
After a bit of digging, it seems that if you have a version of the 2007 Microsoft Office Add-in: 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.
I can't find a knowledge base article on this issue, so I can't be 100% sure that it will solve the problem in all cases, but so far, I'm two for two. Please leave feedback if this helps your situation.