Answer:
The inherent problem in answering this question is that Windows XP does not have a Telnet server included, so there would typically be no purpose in even attempting it. You would need to install a third-party Telnet server (of which there are several) on Windows XP first. After the Telnet server is running, just open a command prompt in Windows Vista and enter:
telnet <ip address of XP computer>
substituting, of course, the correct IP address and leaving out the brackets.