So you've installed the ooooooh shiny Silverlight 5 Release Candidate and now, you can't playback videos protected by PlayReady, maybe you are seeing a nice 'Error 6000: unknown internal error' or some such message?
Here's a workaround: Delete (or rename) C:\ProgramData\Microsoft\PlayReady\mspr.hds, which is the PlayReady database.
You'll lose all your current licenses, but you'll be able to resume operations on your computer ;)
Note: I've tried removing all my Silverlight installations, SDKs, Runtime, Developer Runtime, 'Tools for Visual Studio' and reinstalling only the 4, to no avail. So, you can save yourself time, and lose licenses, knowing that it won't work.