When you have the USB plugged in can you go into the BIOS and set the boot order so that it is first? When you restart the PC with the USB plugged in hit F11 to jump into the boot menu on the BIOS instead of hitting DEL to go into the BIOS settings. Also note you may want to make sure you have the USB drive in a USB 2.0 port on the back of the PC(not the blue ones on the back or front panel).