The short answer is no. XP Professional is a desktop operating system, not a network (server) operating system. There is no way to turn XP into a domain controller (DC).
You are correct that you need a Server OS to create a domain controller. It sounds like the person you talked to is confused on a couple of counts.
NT4 Server required that you choose whether the computer was to be a PDC/BDC during installation. The only way to change the role of the server (either from member server to PDC/BDC or vice-versa) was to reinstall the OS.
Win2K server allows to change the server's role at any time. Installing Active Directory (AD) (by running dcpromo) on any Win2K Server will automatically create an AD domain (if none exists),and turn the server into a DC. If you run dcpromo again, AD is removed, and the server reverts to the role of member server.