When you install a game (this is a legal copy, right?) it should install any components, like the .NET Framework, that are needed. You can also install the .NET Framework by going to Windows Update and choosing advanced search.
Microsoft .NET Framework aka .NET is a programming framework created by Microsoft that developers can use to create applications more easily. Many applications require .NET framework for them to run. You can download .NET Framework 4 here.
SUGGESTION: Upgrade to Windows 8/8.1/7 for more security, features and all.