Allowing random people to stick their dongles into a public usb port like that is just dangerous territory in general. Best defense in Windows against that of course is to disable any form of autorun to prevent software from automatically installing from the device upon connection. Windows XP is the worst possible OS when it comes to this, so Windows 7 on a limited rights user account at the very least is strongly recommended. Some places run software to wipe and re-image the OS every midnight, to undo any possible damage anyone could do that day.
Honestly, the absolute best protection though would be to have the machine for this purpose running linux with a web browser, libreoffice, and other basic tools people would need to do basic online browsing, research, word processing, whatever. Linux is more than enough for these needs. Linux cannot be easily infected with a virus, and even then, not many linux viruses even exist for the attempt.