I know the issue has probably been solved by now, but for reference:
XP should have built-in support for USB keyboards, unless you have a really wonky one (the likes of which I have never, ever, ever seen). Try unplugging and replugging the keyboard after the login prompt appears; failing that, enable 'USB keyboard legacy mode' or somesuch in the BIOS. In layman's terms, it pretends that there is a USB->PS2 adapter installed. Sort of. Unless Windows messes with the USB host and kills everything as usual...