XP Pro won't boot after apparent USB problems?

XP Pro with SP1, Asrock SIS (K7S8X) MB, 1G DDR RAM, two USB 2.0
drives. WaS in the process of copying files from a USB 2.0 external
drive when during the copy all of sudden Windows complained that the
source was no longer accessible and aborted the copy. A check in
Explorer showed that the external USB drive was no longer there.
Unplugging and plugging back didn't work and also another USB 2.0
drive I had no longer showed up.

Rebooted Windows and it was fine but no USB drives would be
recognised. In Control Panel/System/USB devices a double click on
those resources caused Windows to lock up solid. Tried to boot up
Windows in safe mode and it hangs just after loading mup.sys (as well
as asking me if I want to not load xmasbus.sys). I don't load it and
the system still hangs there

Next step - tried a repair installation of XP. Gets to the XP splash
screen with the color bars moving from left to right and then hangs
there.

Finally based on some info in the NB's thought I might have had a USB
hardware failure so disabled USB in the BIOS. Now it boots up and gets
to the installation of XP. Of course this isn't really satisfactory
since I now have no USB ports

Sounds like hardware problems to me but to be sure and before I RMA
the MB, does anybody have any other ideas?

Thanks

Larry