Determine which softwares are using different .net framework runtimes

"Having the different versions of the .net framework installed on a machine is not going to cause any problems. Alot of different applications will be targeting the different frameworks. Trying to uninstall one may end up breaking others.

The largest different is between version 1.1 and 2.0. There were breaking changes made in those two versions of the framework and both of those should be kept installed. Then next step up happened when 3.0 and 3.5 were released. Both of those versions are compatible with 2.0 (in fact are layered on top of) and are optional. The next version to come out will be 4.0 with the next release of Visual Studio, and this will have some new features that will not depend on any of the older versions much like 2.0's release was.

I myself wouldn't worrry about having the frameworks on your machine. They won't hurt anything, and when you download and run programs for your windows machine you won't have to worry about also downloading another framework.

Hope this helps some."