If you want to determine exactly which versions of the .NET Framework are installed, examine the subkeys under the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP
The .NET Framework SDK includes a command-line utility called CLRVer.exe that shows all of the CLR versions installed on a machine.
This utility can also show which version of the CLR is being used by processes currently running on the machine by using the –all switch or passing the ID of the process you are interested in.