I read another thread in another forum about this esact issue. I'll recap the advice given, what the user eventually did, and how it worked out for him. Before we get to that, if you have installed anything new to the computer, disconnect it and try booting without it. Be sure to try safe mode so that you can try to uninstall any software that came with the equipment.
Also, open the covers for your hard drive, memory, etc., and make sure you are not hosting a dust convention. While you're at it, if you have two sticks of memory, put stick A in the slot where stick B is, and vice versa. The reason for doing this is just to make sure the memory sticks have not worked themselves loose. You should make sure your HDD connection is secure as well.
Remember when you are touching the inside components to disconnect the power, remove the battery, and then press the power button. Pressing the power button helps discharge any stored electrons that might be hiding in the circuitry. After you do that, touch a metal part of the frame of the computer before you touch anything. This will help prevent zapping anything with static. Remember to touch the metal frame any time you have been out of contact with the computer for more than a few seconds.
If none of that solves your problem, here is what I found:
It was advised that he repair his existing XP install by doing the following:
A. Disconnect anything connected to your computer that did not come with it. If you have a wireless PCMCIA card, or have stuff connected via USB, or if you are connected to a network via a Cat5 cable, disconnect all of it.
B. You should not lose any data, but if you can possibly connect your HDD to another computer before you do the repair, you may want to back up any essential data files.
1. Boot using the XP CD
2. At the first blue install screen select INSTALL
3. After running through some config gobbledy-gook, another blue screen will appear and notify you that it has detected that XP is already installed. One of the options is to repair the existing installation. Select that one.
4. Hopefully, this will fix the problem for you.
5. You may lose drivers for the various gizmos you have installed since you got the computer. Be sure to have those CD's handy. When you reinstall those drivers, follow the manufacturer's directions. Some want you to install the software before you connect the device. Others want you to connect the device and then do the install. READ THE NEXT STEP
6. As you add each of your additional connected accessories, reboot after each one. If you get your mup.sys problem again, you'll know which one of the buggers to tie to your car bumper.
7. Keep your fingers crossed the whole time!!!