I have the 3.5 and 3.5 sp1 installed with no problem. (Not to mention 1-3 as well)
"The .NET Framework is Microsoft's managed code programming model for building applications on Windows clients, servers, and mobile or embedded devices. Developers use .NET to build applications of many types: Web applications, server applications, smart client applications, console applications, database applications, and more." .NET Framework Developer Center
I can tell you as a developer, the .NET frameworks are excellent tools for developing programs for windows. They make my life easier. As far as installing it, its harmless and if anything will help you in the long run.