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.