A: The .NET Framework 3.5.1 is available in Windows 8 as a feature that can be turned on.

For example, Mouse without Borders ( see my FAQ “Q: Is there a free solution to share my mouse between multiple computers and allow easy copying of data?”) won't work without .NET 3.5.1 being enabled first.

Just perform the following to enable .NET 3.5.1 in Windows 8:
1. Launch Control Panel from the Start screen.
2. Scroll down to the bottom of the Control screen and select "More settings" which opens the Control Panel.
3. Select the Programs category.
4. Under Programs and Features, select "Turn Windows features on or off" (see screenshot below).
5. Scroll down and enable Microsoft .NET Framework 3.5.1 and click OK.
6. Close the control panel.