Some programs you may use require that you have a specific version of the Microsoft .NET Framework installed for them to run correctly. Windows 8.1 has .NET Framework 4.5 installed by default, but an earlier version may be necessary to run some older programs. You can check your version of this framework by using the Windows Registry Editor, which allows you to see a list of the versions and service packs you've installed. You need to use your administrator account to use this tool.
Go to the Windows Start screen and type "regedit" (without quotes) to do a search. Select this utility's name when it appears in the app search results.
Click "Yes" if Windows asks for User Account Control verification to launch Registry Editor.
Navigate to the "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP" entry in the left pane. Each version of Microsoft .NET Framework that you have installed has its own folder underneath the NDP folder that shows its number.
If you need to install a different version of the .NET Framework, you don't have to uninstall any of the ones you already have.
The Microsoft website offers links to download versions 1.0 through 3.5 (see Resources).