When you boot the computer there should be an option for "Select Boot Device" typically on most machines I have seen this is selected by pressing F12.
You should be able to plug in your USB device, boot your computer then hopefully you will see this "Select Boot Device" option and press that key. After that you should be able to see a list of devices which should include your USB key, selecting that and pressing enter should boot the USB device.
Alternatively you should be able to go into the BIOS (generally by pressing either F2 or the Delete key a few seconds after power up) search around for a "Boot" option where you should be able to change the order of devices to make it look at the USB device first. You'll need to remember which order the devices are in and change them back after the install is complete.