Rick Strahl had some problems with his Tech Preview - it killed ASP.NET and the IIS Management Console. Now, this hasn't happened to everyone (in the blogs I've been checking, most people have been fine), but if it happens to you, we've worked out the solution. I'm taking the key steps from Rick's post if you are seeing the same issue - but go read the whole thing...
Find the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET key look for errant key entries that include braces and other junk. The bogus keys look like this:
Clear out the key names to read proper version numbers. It looks like .Net 2.0 installs two different versions of the CLR:
Inside of those keys make sure that the DllFullPath and Path values point at the correct .Net Framework directories in your Windows installation. For example: