XP Home boot hangs at mup.sys

Trying to help a friend with what turns out to be a common problem (after researching on the www). Windows XP Home running on a Dell Dimension, nothing sepcial about it or its configuration. All of a sudden decides to go into a booting loop, and if trying in Safe Mode, occurs at the same time each loop – as mup.sys device driver loads.
There is a lengthy forum thread about it on Computing.net (http://www.computing.net/windowsxp/wwwboard/forum/30471.html)

Next time I visit my friend, I will try Disabling mup.sys from the XP Recovery Console, but I’d really like a more satisfying explanation and fix, if anyone can offer one.

Thanks