Version 4.5 of the .NET Framework is included with Windows 8 and is probably already installed on your Windows 8 PC. Some applications require specific versions of the .NET Framework and will sometimes prompt users to download and install the required version. If the application doesn't offer to install the proper version of the.NET Framework, it can easily be enabled via the Windows Features interface.
Open the Control Panel
Under Programs and Features, click on Turn Windows features on or off
Select .NET Framework 3.5 (includes .NET 2.0 and 3.0)
Click Download files from Windows Update