Does your monitor have two inputs? - A VGA and a DVI?
If so you connect each computer to the different inputs, and can switch between them on the monitor.
This is exactly how I do what you want to do. My computer is on the DVI input - all others that come my way go onto the VGA input. I keep a cable permanently in the VGA input, and merely have plug into the "secondary" computer.
If you don't have two inputs, then you need an external switch, as or similar to the above.
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