Windows XP has a nice feature where you can log on multiple times as different users concurrently, while at the same machine (switch user). It's a nice feature if you want to test your application as a lower privilege user, for example (or you could just use "RunAs"..)
Windows Server 2003 does not have this (It's a server, is that a good enough reason? not sure..) but you can emulate this behavior by doing a remote desktop connection to the local machine:
press start-Run, type "mstsc" and type the name of the local machine in the dialog box. you can then log on as whoever you want while running as the original user.
I know - simple, but I just figured this out today...