Before I forget, there might be one further problem that you may run into, and that is that windows may simply refuse to install to a usb device, even assuming that you can boot to one.
Something to bear in mind is that usb devices are by their very nature removable, therefore any OS that is installed to such a device would need to be able to reconfigure itself to virtually any hardware to which it was attached, therfore I suspect that it will not be possible to install any windows OS on such a device.