Thank you bionik after talking to you I researched about BIOS and came across another forum which solved my problem with the information below :)
Step 1 - the trick - make sure your bootable USB stick is in before you turn the Duo on
Step 2. - on startup while in Dell's splash screen, select f2 to enter the BIOS screens.
Arrow over to the Boot menu and change your boot priorities so that USB runs first (select f5 with the USB highlighted and it will step the USB to first priority), HDD second. Save changes and exit, which will begin your reboot...
I run Ubuntu from a USB stick very regularly - the MoBo happily supports boot from USB so you should have zero issues
If you need a more detailed set of instructions let me know - I write tech manuals for a living and will happily draft a little walk-thru if it would be helpful to you?
PS - once the bios is set, to boot from the HDD first, select f12 from teh Dell splash to enter the boot options menu and select HDD instead