Windows 8 includes the .NET Framework 4.0 installed by default but, older software which require .NET Framework 3.5 or 2.0 can’t be run instead you have to download it from Windows Update. If, you are running a software which requires the .Net Framework 3.5 or earlier then, Windows 8 will prompt you to download it from Windows Update but, some programs may not ask you to install the .NET Framework and still they won’t run. In that case you will have to install .NET Framework manually. This tutorial will guide you through the process of installing the .Net Framework manually in Windows 8.
Just follow the instructions below and you would be able to install .NET Framework easily in minutes.
Instructions:
Note: You would need an active internet connection to download files from Windows Updates.
• First of all, open up ‘Control Panel‘ and select ‘Windows Updates’ from the icons view.

• Your ‘Windows Updates‘ window would open. Here press ‘Installed Updates‘ from the bottom left corner of the window.

• In the next window select ‘Turn Windows features on or off‘ from the left column.

• Another pop-up window would open. Here expand the .NET Framework 3.5 (includes .NET 2.0 and 3.0) tree.

• After expansion, mark both sub trees (they should look like the screenshot above) and press OK.
• Now, Windows will search for the required files and then, ask you for permission.

• Grant permission by pressing ‘Download files from Windows Updates‘.
• Your files will start downloading. It may take some time for the download to complete depending on your internet connection speed.

• After download and installation, you would be told that ‘Windows completed the requested changes‘.

• Congratulations! You .NET Framework has been successfully installed.
Tip: You may need to re-open the software to run it if, it requires .NET framework.
just in case you can't tell the source: windows8core