After a bit of searching around — back to 2003 (the problem is an old one) we find that it may not relate to mup.sys (Multiple UNC Provider driver — which handles network addresses like \\server123\publicfolder etc). The problem might be with the next driver to be loaded.
Another clue: when attempting a Normal-mode XP boot, the PC’s beeper gives intermittent high-pitched beeps and then a continuous beep. These beeps are low-volume, but quite annoying; I’m guessing around the 4000 Hz mark).
That may indicate a cpu, memory or motherboard problem… But why does the more sophisticated OS Win 7 work ?