If you already have .Net 4.0 installed and install a program that brings in an earlier version of .Net, or manually install an earlier version of .Net afterwards, you may get an exception page that matches the title of this article.

This is a simple fix, outlined in detail in MSDN article referenced at the bottom.

Navigate to:

%windir%\Microsoft.NET\Framework\v4.0.30319

%windir%\Microsoft.NET\Framework64\v4.0.30319 (on a 64-bit computer)

Run:

aspnet_regiis.exe /iru

That’s it!

http://support.microsoft.com/kb/2015129