Windows USB drivers have always allowed USB booting, it's Windows itself that doesn't allow it. Windows 8 allows official USB booting but only when you make a portable version (Windows To Go) that I think is restricted to the enterprise version only. There are several guides that explain which INF files to edit to have windows XP/Vista/7/8 boot from USB, good luck with the other drivers (SATA/video/audio/wifi/etc.) though.
do usb mass storage boot drivers exist for any modern version of windows?