Telnet is now made an optional component in Vista (and Longhorn Servers). This means if you type telnet in command shell, you will be out of luck.
To get this usefull tool back, you can use the software explorer or Click Start, ControlPanel, Programs, and then Turn Windows Features on or off. In the list, scroll down and select Telnet Client. Click OK to start the installation.
As time has passed, fewer users use telnet. Thus, to decrease the foot print as well as the attack surface, we decided to make it an optional component.
for more infos see this MS website .