If you get the error "This document could not be registered. It will not be possible to create links from other documents to this document" when you try to open an office document, then try the following:

(You might also get errors about the Windows Installer service not being able to start)

(Also, if you have FrontPage 2003, you might have problems opening the program altogether)

- Go to Start > Run > services.msc (or Services under Administrative Tools)
- Go to the DCOM Server Process Launcher and set it to Automatic. You can try to start the service there also, but you may have to reboot.

Now try to open an office document. Doing the above fixed all those problems for me.