It's largely dependant upon the motherboard and bios you have in your computer.
Boot the machine up and hit the Del key or whatever to access the BIOS, there should be a message on hte bottom of your screen telling you how to get into BIOS or Settings or somesuch.
GO to teh Advanced settings and see if there is an option that says something along the lines of "Boot from alternate devices" and make sure that is enabled.
Esc, and save and exit the BIOS settings.
You should be able to boot up off the USB drive.