FlashBoot is an ultimate tool for making USB devices bootable.
FlashBoot can install fully-functional Windows 8 to USB thumbdrive (so you can boot Windows from USB and carry your OS, your apps, your documents, your browser in the pocket, run it in public library, in internet-cafe, work-from-home etc).
Also, FlashBoot can convert Windows XP/Vista/7/8 installation CD/DVD to bootable USB disk, useful for Windows installation on netbooks and other devices without CD/DVD drive. FlashBoot supports USB flash thumbdrives and USB hard disks.
FlashBoot home page: http://www.prime-expert.com/flashboot/