To find out what versions of .NET Framework you have installed on your computer, go to C:\Windows\Microsoft.NET and among the files you will see a few folders. If you have folders of the form v1.0.xxxx in there, you have .NET Framework 1.0 installed, if you have folders of the form v1.1.xxxx, then you have .NET Framework 1.1 installed, and finally if you have folders of the form v2.0.xxxx then .NET Framework 2.0 installed. Obviously, since you can have more than one version of .NET Framework on your computer, you can also have multiple folders of the form vx.x.xxxx.

If you were expecting a solution for retrieving the available versions programatically in your Windows application, you can either check for the availability of the folders, or for the appropriate registry values. However, if you are using the .NET Framework itself for the application in which you wish to retrieve the versions running on the targeted computer, you should use the method specifically designed for this purpose, which is covered in the following article:
How to retrieve the installed ASP.NET version