See the question and my original answer on StackOverflow

Here is a good link for these kind of errors: Microsoft .NET Framework 4.0 RC and Visual Studio 2010 RC Available Now - Support

I quote:

If you are having problems installing .NET 4.0 or Visual Studio 2010, please visit support forums.

You may find solutions to common problems right away, or the community – including Microsoft employees – will be glad to assist you. If you are having installation problems, please download and run the log collection utility. This creates a file in your %TEMP% directory that you can upload and paste the link in any correspondence.