Often in logon scripts you test for versions and releases

Official Windows operating system version numbers.

Operating system	Version number
Windows 7 Windows Server 2008 R2	6.1
Windows Vista Windows Server 2008	6.0
Windows XP 64-Bit Edition Windows Server 2003 R2 Windows Server 2003	5.2
Windows XP	5.1
Windows 2000	5.0

Windows build numbers.

Windows 7 Windows 2008 R2	7600 7601(SP1)
Windows Server 2008	6001 6002(SP2)
Windows Vista	6000 6001(SP1) 6002(SP2)
Windows 2003 Windows XP (x64)	3790
Windows XP	2600
Windows 2000	2195
Windows NT4 / NT4 TS	1381

There
is
an
exhaustive
list
here:
http://en.wikipedia.org/wiki/Windows_NT