Look at the @ProductType macro.

OS type. Possible values:
"Windows 95"
"Windows 98"
"Windows Me"
"Windows NT Workstation"
"Windows NT Server"
"Windows NT Domain Controller"
"Windows 2000 Professional"
"Windows 2000 Server"
"Windows 2000 Domain Controller"
"Windows XP Home Edition"
"Windows XP Professional"
"Windows XP Professional Tablet PC"
"Windows XP Media Center Edition"
"Windows XP Starter Edition"
"Windows Starter Edition"
"Windows Server 2003"
"Windows Server 2003 Domain Controller"
"Windows Server 2003 R2"
"Windows Server 2003 R2 Domain Controller"
"Windows Vista Home Edition"
"Windows Vista Professional"
"Windows Vista Professional Tablet PC"
"Windows Vista Media Center Edition"
"Windows Vista Starter Edition"
"Windows Vista Server"
"Windows Vista Domain Controller"