telnet software for windows does indeen exist....
If you have XP pro then you can just turn on the telnet server that come with windows, (cheers bill, -not only free but also included)
start > control pannel > administrative tools > services
find telnet tell it to start automatically.
If he has xp home then I'd say a good telnet server is the kpdm telnet server (again free)
just a bit of clarification...
you can't telnet to his machine unless he has a telnet server running that allows you access.
him running a telnet client won't help. it has to be a server.
after this he's going to have to allow port 23 on his firewall to be open.
last but not least, after all of this you're going to need to have a username and password to logon to the computer.