If your desktop bios doesn't allow booting from the usb flash drive, then...

If your desktop bios doesn't allow booting from the usb flash drive, then your not going to be able to install Ubuntu from it. Have you checked to see if there is a bios upgrade that you could install that would give you the ability to boot from the usb stick?
Other options are quite a bit more complicated, and depend on how old your hardware is, and what other limitations it has.

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