I know it's an old question but I thought these were useful enough to put here for people searching.
This first one is a simple batch way to get the right version.  You can find out if it is Server or Workstation (if that's important) in another process.  I just didn't take time to add it.
We use this structure inside code to ensure compliance with requirements.  I'm sure there are many more graceful ways but this does always work.
:: -------------------------------------
:: Check Windows Version
:: 5.0 = W2K
:: 5.1 = XP
:: 5.2 = Server 2K3
:: 6.0 = Vista or Server 2K8
:: 6.1 = Win7 or Server 2K8R2
:: 6.2 = Win8 or Server 2K12
:: 6.3 = Win8.1 or Server 2K12R2
:: 0.0 = Unknown or Unable to determine
:: --------------------------------------
echo OS Detection:  Starting
ver | findstr /i "5\.0\."
if %ERRORLEVEL% EQU 0 (
echo  OS = Windows 2000
)
ver | findstr /i "5\.1\."
if %ERRORLEVEL% EQU 0 (
echo OS = Windows XP
)
ver | findstr /i "5\.2\."
if %ERRORLEVEL% EQU 0 (
echo OS = Server 2003
)
ver | findstr /i "6\.0\." > nul
if %ERRORLEVEL% EQU 0 (
echo OS = Vista / Server 2008
)
ver | findstr /i "6\.1\." > nul
if %ERRORLEVEL% EQU 0 (
echo OS = Windows 7 / Server 2008R2
)
ver | findstr /i "6\.2\." > nul
if %ERRORLEVEL% EQU 0 (
echo OS = Windows 8 / Server 2012
)
ver | findstr /i "6\.3\." > nul
if %ERRORLEVEL% EQU 0 (
echo OS = Windows 8.1 / Server 2012R2
)
This second one isn't what was asked for but it may be useful for someone looking.  
Here is a VBscript function that provides version info, including if it is the Server (vs. workstation).
private function GetOSVer()
    dim strOsName:    strOsName = ""
    dim strOsVer:     strOsVer = ""
    dim strOsType:    strOsType = ""
    Set objWMIService = GetObject("winmgmts:" & "{impersonationLevel=impersonate}!\\.\root\cimv2") 
    Set colOSes = objWMIService.ExecQuery("Select * from Win32_OperatingSystem") 
    For Each objOS in colOSes 
        strOsName = objOS.Caption
        strOsVer = left(objOS.Version, 3)
        Select Case strOsVer
            case "5.0"    'Windows 2000
                if instr(strOsName, "Server") then
                    strOsType = "W2K Server"
                else
                    strOsType = "W2K Workstation"
                end if
            case "5.1"    'Windows XP 32bit
                    strOsType = "XP 32bit"
            case "5.2"    'Windows 2003, 2003R2, XP 64bit
                if instr(strOsName, "XP") then
                    strOsType = "XP 64bit"
                elseif instr(strOsName, "R2") then
                    strOsType = "W2K3R2 Server"
                else
                    strOsType = "W2K3 Server"
                end if
            case "6.0"    'Vista, Server 2008
                if instr(strOsName, "Server") then
                    strOsType = "W2K8 Server"
                else
                    strOsType = "Vista"
                end if
            case "6.1"    'Server 2008R2, Win7
                if instr(strOsName, "Server") then
                    strOsType = "W2K8R2 Server"
                else
                    strOsType = "Win7"
                end if
            case "6.2"    'Server 2012, Win8
                if instr(strOsName, "Server") then
                    strOsType = "W2K12 Server"
                else
                    strOsType = "Win8"
                end if
            case "6.3"    'Server 2012R2, Win8.1
                if instr(strOsName, "Server") then
                    strOsType = "W2K12R2 Server"
                else
                    strOsType = "Win8.1"
                end if
            case else    'Unknown OS
                strOsType = "Unknown"
        end select
    Next 
    GetOSVer = strOsType
end Function     'GetOSVer