supportedRuntime Element
Specifies which versions of the common language runtime the application supports. This element should be used by all applications built with version 1.1 or later of the .NET Framework.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpgenref/html/gnconsupportedruntimeelement.asp
Targeting a .NET Framework Version
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpcontargetingnetframeworkversion.asp