.NET Framework is used to provide various services for applications but it is not possible to say whether or not you have any of them installed on your PC.
Any or all of those versions my be needed. That is to say .NET 4.0 does NOT replace or contain the functionality of the previous versions.
If you delete any or all of the versions of the .NET Frameworks then some (or none) of your applications will cease to run and will call for you to reinstall.
T
October 2010