I recently tryied to boot a USB memory stick in a new Dell OptiPlex 760 USFF, and I had to do the following to get it to work.
I downloaded a Dr. Dos 7.1 disk image, and installed it to a USB floppy.
Then I inserted the USB drive, and rebooted.
In Dos ( on the floppy ) I did a fdisk, selected the USB disk, and created a Primary Dos partition.
Then I rebooted and did a format C: /S
This allowed me to create a bootable USB drive, that works, and I can insert use it in a XP device also if needed
I then put the drive in my system, and copy the Windows CD of choice to the drive, and do a Dos Unattended windows Install. ( more on this as time permits. )