If this is a W2K server, you go to Control Panel - Add/Remove
programs - Add/Remove windows components and change the mode.
Be aware that this implies a couple of things:
* you will need a TS Licensing server and valid TS CALs
* before switching mode, you need to uninstall all applications that
will be used in multi-user mode, and after the switch, you will have
to re-install them (while the server is in "install mode"