Using a Laptop
If you want to use your own portable computer with the data projector, you will need to connect it up to the AV system using the spare cable (i.e. the one that isn't connected to the PC). This will cause the auto-switcher to switch to your laptop until such time as it's disconnected and the system returns to its default state of using the PC.
Disclaimer: Some more esoteric laptops can be troublesome and whilst we will do what we can to help you, if it's a machine that's unfamiliar to us we can't guarantee to be able to make it work.