If you are using Windows 2008 R2 operating system, you may receive the following error while installing Visual Studio 2008 or .NET Framework on your server:
You must use the Role Management Tool to install or configure Microsoft .NET Framework 3.5

This is because Windows 2008 R2 comes with pre-installed .Net Framework 3.5.1. You just require to enable this feature from “Server Manager”.

Using Role management tool, you can easily enable .Net 3.5 framework on your web hosting server or development server.
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Thank you for this information.. I was playing with my server to activate .Net Framework 3.5 but I got the above message. Thanks for providing information to enable .Net framework 3.5.