You may be required to provide your Windows CD.
Go to Windows Explorer and click on Uninstall or change program in the top menu bar

In the menu on the left click on Turn windows features on or off

Wait for the window that opens to show all the contents, then scroll down to Telnet Client and make sure it is checked.
Then click OK. This will now install telnet on your PC.

To verify that telnet was installed, go to Command Prompt in the Start Menu

and type telnet{Enter}, to get the following output.

For which operating system is this? It looks like Windows. What version/s of Windows?