Telnet's no good really.. I'm not sure if there's an SSH server for windows, but if there is use that over telnet any day.. Telnet/SSH is kind of useless for a windows desktop anyway, unless suddenly telnet has a gui I'm not aware of.. or you've started running things in text mode.. if you just want to be able to access your files, set up an FTP server.. if you want to be able to run applications and stuff graphically, I'd recommend RealVNC..
Oh, and to answer your question, if you're really dead-set on telnet, it's port 23