The .Net 3.5 install is an optional install found in the Turn Windows Features On or Off section of the control panel.
- On the Start screen, type and then choose Control Panel
- Choose Programs, and then choose Turn Windows features on or off
- Select the .NET Framework 3.5 (includes .NET 2.0 and 3.0) check box
- Follow the prompts to install .Net 3.5 and then reboot