Errors after Installing Dev10 Beta2 over top of Beta1 on Vista RTM or SP1
There is an issue upgrading .NET 4.0 from Beta1 to Beta2 only on Vista RTM and SP1 as well as Windows Server 2008 SP1. This is due to a known issue that was fixed in Vista SP2, Win7, and Windows Server 2008 R2. It is not an issue on XP.
There is a document available here for how you avoid the problem and directions for how to recover if you are already in a bad state.