Hey guys,  I have a Dell E521 with windows 7 Pro (64 bit) I'm trying to boot from the USB to use a bootable copy of UBUNTU, However, I cant get the PC to boot into the USB.  I went into the bios and changed the boot sequence, but it still boots from the HD.  I tried using the front USB ports and the back USB ports but it still boots from the main HD.

What I'm a doing wrong, . . ?  Am I missing something else to change in the bios?  Anyway, help would be appreciated.  Thanks, Ray