By default, Vista doesn't install the Telnet client.
The client is for text-based communication with remote systems.
You can install it by following these steps:

1. Click Start then select Control Panel.
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