The SQL Server 2008 install requires the .NET 3.5 framework. Changes in Windows 7 can prevent SQL Server 2008 from installing the .NET 3.5 framework pre-requisite. Instead you may need to install the framework separately.
In Windows 7, the .NET Framework 3.5 is included as a Windows component. To avoid this installation failure, you must turn on the .NET Framework 3.5 from the Windows Features before you run the SQL Server 2008 RTM installation.
1. Download the .NET 3.5 Framework and save the dotnetfx35setup.exe on the local drive. (http://download.microsoft.com)
2. Using Explorer access the File Properties | Compatibility tab and enable compatibility mode for Vista SP1.
3. Run donnetfx36setup.exe as ‘ADMINISTRATOR’ to complete the install.
4. Execute SQL Server setup again to install SQL Server.
Bob Dorr – Principle SQL Server Escalation Engineer