If you're upgrading to the latest version of Avalon (I really should start to call it Windows Presentation Foundation, I know, but I want to make sure that search engines find blog entries like this for both the codename and the final technology name), you might find that there's some old cruft on your machine lying around that causes you problems during the installation process. Dan Mohr, an SDE/T on the team explains further:
"With the PDC bits, the Windows Presentation Foundation installer does not force overwrite existing files. This may cause problems if you have files from previous Windows Presentation Foundation builds left on the machine. This problem is compounded by a bug in some earlier versions (before the May CTP) that caused the shared DLL reference counts in the registry to not be correctly decremented during uninstall leaving some shared files on the machine."
Welcome to the world of pre-beta software - if you think this is painful, you should try running daily builds 🙂