Here is a list that matches some of the operatingSystemVersion attributes with their meaning. I like checking against operatingSystemVersion better than operatingSystem. I like numbers better I guess.
5.0 (2195) 'Windows 2000 Professional
5.1 (2600) 'Windows XP Professional
6.0 (6000) 'Windows Vista
5.0 (2195) 'Windows 2000 Server
5.2 (3790) 'Windows Server 2003