.NET 3.5 error when installing SharePoint Designer 2010
If you are installing SharePoint Designer 2010 on your Windows Server 2008 R2 environment, the installation may prompt you to install .NET 3.5 before installing SharePoint Designer 2010. The message will nicely present you with a download link, but when installing from there you will get an error, telling you to use the Role Management Tool to install .NET 3.5.
The reson for this is, that your Windows Server already comes with .NET 3.5.1, but since you are getting the initial error from the SharePoint Designer install, it is not enabled.
The .NET 3.5.1 sit on your server as a feature, that you can easily enable using the Server Management application. After you have done this, you can rerun your SharePoint Designer 2010 installation and it will finish smoothly.