Hi Mahesh,
Both Windows 7 and Windows Server 2008 R2 have the "VersionNT" as "601" (in other words 6.1) and "WindowsBuild" greater than 7100. So, it is very difficult to identify the exact OS (Windows 7 or Windows Server 2008 R2).
See this link for more details: http://msdn.microsoft.com/en-us/library/aa370556(v=vs.85).aspx
It seems that Adobe Analytics identifies "Windows NT 6.1" as Windows 7 OS only (even for Server 2008 R2).
OS can be captured in any analytics variable but again it would be difficult to detect which one is the exact (Windows 7 or Windows Server 2008 R2) reason being the same version i.e. 6.1.
Here can be an EXCEPTIONAL case:
If you're tracking a website which is internal to your firm and you're sure that the only OS being used (between the two in question) is Windows Server 2008 R2, then you can capture the OS in a prop/eVar like this:
if (navigator.appVersion.indexOf("Windows NT 6.1")!=-1)
{s.prop3="Windows Windows Server 2008 R2";}
Hope that gives you some insight.
Regards,
Ryan