http://blogs.msdn.com/b/sqlexpress/archive/2009/08/10/troubleshooting-net-2-0-3-5-installation-errors.aspxThe installation of .NET 3.5 can fail for several reasons. Sometimes the .NET 3.5 install fails and sometime SQL Server Setup fails to detect .NET is already on the machine.   If you view the %temp%\sqlsetup.log you can determine whether the .NETen-USTelligent Evolution Platform Developer Build (Build: 5.6.50428.7875)