Hi, I had to make an account just to be able to answer your question
. Insert your USB device and when the system boots and you see the bios screen keep hitting F8. You will see a boot list and your usb device will be listed there. Just select the device and hit enter. The system will boot from USB. I hope it will work for you just as it did for me! If you have any more questions, I would be glad to help you out.
Just a side note: If you want to install an operating system I recommend inserting your usb device in one of the usb ports at the back of your case. The front panel usb ports work fine when booting but windows 7 for example tends to give an error message when the usb is not inserted in a high-speed usb port.
Omid