How to determine the version of .NET framework in your system - One

Locate the following folder (you can use Windows Explorer):
%systemroot%\Microsoft.NET\Framework

The following folders contain the released versions of the .NET Framework:
Folders with vN.N.NXXXX format, contain beta or pre-released versions.
* v3.5
* v3.0
* v2.0.50727
* v1.1.4322
* v1.0.3705

Find Mscorlib.dll inside each of the above mentioned folders, right click on it, click on Properties, click on [Details] tab and check [Product version]

To find all the version numbers, look at the following URL in [Microsoft Help and Support] web pages:
http://support.microsoft.com/default.aspx/kb/318785

How to determine the version of .NET framework in your system - One