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By default, Vista doesn't install the Telnet client.  Often used for debugging purposes (network connection problems / SMTP diagnostics etc). Install it by following these steps:

  1. Open Control Panel from the Start Menu.

  2. Select Programs and Features.

  3. Select Turn Windows features on or off.

  4. Select the Telnet Client option.

  5. Click OK.

  6. A dialog box will appear to confirm installation. The telnet command should now be available.

Note that it might take some time for this to complete.