I ran into a problem today where my Visual Studio 2008 install gave me the “The application cannot start” error after I had installed the RC of VS 2010. After poking around, I found this post which helped me out a lot.
I had to copy these files from “C:\Program Files (x86)\Common Files\microsoft shared\MSEnv”:
and paste them into “C:\Program Files (x86)\Microsoft Visual Studio 9.0\Common7\IDE”
Then everything worked great. No registration was needed.