A Safe Way to Get the User to Check if the .NET Framework is Installed

My method of choice has always been to look at the Windows folders listed under C:\Windows\Microsoft.NET\Framework.

Each framework version has its own sub-folder:

14/03/2011  23:11    <DIR>          v1.0.3705
14/07/2009  03:37    <DIR>          v1.1.4322
14/03/2011  23:11    <DIR>          v2.0.50727
14/07/2009  05:56    <DIR>          v3.0
14/03/2011  23:11    <DIR>          v3.5
28/04/2011  14:44    <DIR>          v4.0.30319

You also can easily get this from a command prompt by dir %windir%\Microsoft.NET\Framework\ /AD.