The/Your problem is not mup.sys - it is what comes after mup.sys that is causing the system to hang. Trouble is, that is sometimes hard to figure out.
It is too bad that sometimes folks spend a long time trying to figure out how to replace mup.sys when that is not the problem at all (I did that once).
I usually find the next thing that wants to load to be the keyboard/mouse driver, usually:
That does not mean that there is a problem with that file - XP just can't process it anymore for some reason.
Sometimes, you have to wait a long time looking at mup.sys in a Safe Mode boot for XP to either figure it out or move on. I would give it a generous amount of minutes before giving up on it.
Describe the mouse, keyboard and any other external USB device environment. Wired, wireless, USB, PS2, printer, etc.
If you have updated any drivers lately, from the F8 Boot Options menu, choose:
Last Known Good Configuration
LKGC is a snapshot of the configuration Windows used/saved on the last successful boot. LKGC is the thing to choose after some driver update keeps your system from booting. Any driver updates lately? If that gets you going, you can undo or fix whatever driver you updated last.
If you think you are getting a BSOD and just can't see it when the system reboots, from the F8 options menu choose:
Disable automatic restart on system failure
Then you can see the BSOD and what it is telling you.
My first suspect is a USB mouse and/or driver change and you don't need a mouse to boot and choose Safe Mode since the keyboard is available. Will your system boot in Safe Mode with no mouse?
You can also unplug everything except that keyboard and see what happens - no mouse, no printer, no etc. Just the absolute minimum to see if you can narrow down the culprit.
We can figure out exactly what it is later after getting more information.
Edited by joseibarra, 30 May 2010 - 11:28 AM.