How do I find out which variant of Windows (server or workstation) I'm running on?

You can find out if your system is a server or a workstation by reading the registry and checking this key:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\ProductOptions

Look for the value "Product Type". There are three possible values :

"WinNT"   workstation
"ServerNT"   server
"LanmanNT"   domain controller