Any number of things can produce a failure to boot. Unless someone else has a better idea I would boot from the Windows? install disk and first do a chkdsk to try and repair any damage. If that doesn't work you could try FIXBOOT & or FIXMBR. If that doesn't work then a re-install of the O/s seems the most likely route to take. Before doing any of this just make sure that all your components are seated properly.