When you can crash any PC in under 5 minutes of use, without even trying.
When you would consider buying all of your family and friends macs and isights just so you can video chat with them.
When you feel the panic coming on after you have to instruct a PC user to restart by clicking "Start" "Shut down" "Restart" ...
When using a PC, you find yourself turning to your wintel using workmates and saying, innocently "umm, does windows have a web server built-in?"
When you could pick your computer out of a line up of exactly the same model from 50 metres back.