Windows XP only allows for a single (console) instance of an interactive logon. Windows Server 2003, without terminal services licensed and enabled, will allow for an additional two remote connections. Occasionally, usually because users forget to logout from remote desktop, but occasionally because remote desktop doesn't reuse preexisting connections, you may be prompted with:

The terminal server has exceeded the maximum number of allowed connections.

In order to connect, you may logout users from existing connections.  To do so, there are a couple of command prompt commands you must know:

qwinsta /server:servername 

This will list all active connections (including via the console). You can identify who has the open connection via the username column. Find the ID of the connection you wish to terminate, and pass it into the following command:

rwinsta /server:servername ID