http://answers.microsoft.com/en-us/windows/forum/windows_other-windows_update/problem-installing-net-framework-with-windows-7-64/076243f8-190f-4ad2-9237-ad574ee25cbd

"For Windows 7, .NET Framework 3.5 with SP1 is shipped with it as one of OS component.
For .NET Framework 3.5 with SP1, it includes .NET 2.0 with SP2 and .NET 3.0 with SP2 as prerequisite.
This means, you have the .NET Framework 2.0 SP2, 3.0 SP2 and 3.5 SP1 plus a few post 3.5 SP1 bug fixes on Windows 7. They are OS components. We don't need to manually install it."