Try booting the USB drive on another computer that you know can boot from USB; are you getting the same error? Did you also check the contents of the drive you installed to after UNetbootin installed; were the contents of the Ubuntu ISO file, and the files syslinux.cfg, ubnkern, ubninit, and ldlinux.sys (you may need to use "show hidden files" to see them) in the top directory of the drive?
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