This information is provided with no warranty of any kind-- USE AT YOUR OWN RISK. It is provided here because an answer was not found on the web that was satisfactory short of "reinstall windows".
HISTORY
Had been using XP MCE (same issue can happen with PRO, etc.) and one day it crashed and rebooted. The user had been faithfully installing all the windows updates over the years, etc., etc. When it restarted it crashed/rebooted at MUP.SYS:
C:\WINDOWS\SYSTEM32\DRIVERS\MUP.SYS
POSSIBLE SOLUTION
We tried booting into safe mode, we tried "last known good", etc., etc. None of it worked. We looked on the web and nobody had good answers. Just guesses. Here's how we fixed it:
Boot with a different CD. Can be anything as long as it will let you mount the C: drive (ours was NTFS)
Cd into C:\WINDOWS\SYSTEM32\DRIVERS and rename MUP.SYS to DISABLED.MUP.SYS
Reboot
Once we did that it came fine and went all the way to SP2PORT.SYS and did the same thing.
We therefore repeated the steps above and renamed SP2PORT.SYS to DISABLED.SP2PORT.SYS and now Windows XP MCE boots fine and is "stable" again. The downside is that although all of the network services work, it can no longer see other local windows computers and mount filesystems off of them. We can access the Internet, etc., nameservers, etc., all fine and if you have a single-pc setup this would be fine.
IF YOU STILL need access to a remote CIFS fileserver all you have to do is rename DISABLED.MUP.SYS back to MUP.SYS and DISABLED.SP2PORT.SYS back to SP2PORT.SYS. Do not restart. Simply access your volume as your normally would and it will probably work. Your system may crash again upon reboot and you'll have to re-disable these to get it to load.
MUP.SYS is microsoft's remote filesystem interface and if it doesn't get loaded it doesn't load other drivers and you can't access remote CIFs shares. If you have a single-pc that doesn't need access to a fileserver on your local network then this fix might just work for you.
