In windows xp and lower versions telnet port i.e. 23 is open by default but there comes a need to open it manually in case of windows 7.
1.Go to control panel
2.programs>programs and features>Turn windows features on or off
3.There just check the Telnet Client button and thats all.
4.Restart the system.
NOTE: telnet is used for remote login so i advise to keep it open only if you are using it , not just for fun,it can be a wrong decision and you can be very close to be hacked!!!!