How to fix an XP\Win 2000 System that freezes after loading mup.sys while booting

Symptoms: 1. When booting in normal mode my windows XP system hangs after displaying the XP logo and progress bar. A black screen appears and then..
or 2. When booting windows XP in safe mode the last driver that shows loaded is mup.sys. Then...
or 3. Instead of XP freezing at mup.sys windows reboots itself repeatedly just after that driver loads.

Then there is a flurry of HD activity and all the LED's on the kybd light up and then go out. The Keyboard is locked up and dead at this point. There is
some minor HD activity with maybe 3 or 4 blips. Usually the mouse laser cycles at this point but goes out. You have the classic Windows XP frozen at
mup.sys. The variation on this is that instead of XP being frozen at mup.sys it actually resets itself after loading that driver in an continuous loop.
Both cases can be considered the same issue as we address them here.

Many users have run into this problem and there are many different causes and just as well even more solutions. This shows as a hang at mup.sys
because after mup.sys loads, Windows then goes into a more complicated portion of its boot process and this is where the actual hang occurs.

So, What happens after we see the mup.sys driver load on a safe mode boot?

The windows OS is looking in the registry, executing PnP (Plug and pray) and ACPI routines checking the components and resources found.
It then starts these components possibly causing more draw on the power supply. Consider here the load of non-self-powered USB devices.

If there are any unrecoverable problems here, that will cause the system to hang and it will look the same. - The system hangs during a
safe mode boot after loading mup.sys!

The main reasons for this Windows XP or Win 2000 boot hang or alleged mup.sys issue are:

Hard disk failure or corruption
A corrupted registry or registry hive
New hardware has been installed but not did not completely "Register or re-Register" correctly
New hardware has been installed but it is faulty or failing
The new hardware's driver or windows itself has been compromised (Disk data corruption or by a virus) or (rare) needs to be updated
The power supply is marginal in output or failing (Common per user feedback)
BIOS\ESCD\Motherboard chipset driver conflict with a component, its driver, or its registry data
Existing hardware including the motherboard may have failed in a specific way but not catastrophically.

In my experience,  I have never found that the mup.sys driver is actually the cause of the problem but replacing it with a known good uninfected version
won't hurt if just to exclude that possibility. Also, disabling the mup.sys driver by using the recovery console normally does not help either. Windows will
then just hang at the driver that loads prior to mup.sys.
If you are curious and want to know more about this driver check out the What is MUP.SYS and what does it do? section below.

NEW !! Be sure to check our new User Feedback on mup.sys resolutions page. Real life fixes from our readers.

Even more useful resolutions recently added in February 2010. Thanks to all for the valuable feedback!

Here are some troubleshooting suggestions with the more frequent causes and resolutions colored dark blue:

ESCD corruption, Try resetting ESCD and/or changing ACPI setting in the BIOS. If there is no specific way to reset the
ESCD data via the BIOS you can force this by reflashing. For added value check to see if there is a BIOS update available first.
Here is a link to a Phoenix and Award BIOS FAQ that provides additional BIOS related information and troubleshooting steps
If your system is overclocked, Try reverting to the original standard spec.'s
If you have USB devices connected, including any hubs, try removing them all before you boot
If you have devices connected to the serial or parallel ports, Try disconnecting them, again before you boot
If you added any new hardware recently, including PCI,PCI-E, ISA cards, Or memory DIMMs Try removing them one at a time, reboot
and see if this allows the system to boot correctly
If you have access to a DMM (Multimeter), Check all the power supply voltages and try to watch each as you try to boot. If they are more
than 7% below normal and\or your PSU is over 5 years old, less than 350W and you have a 3D gaming graphics card with 128MB
or more of graphics memory. Consider replacing the power supply with an upgrade of 500W or better.
For more information on troubleshooting PC Power Supplies see our article on ATX PSU Troubleshooting
If you have PCI, PCI-E or AGP Graphics card and also have a video connector on your motherboard try taking out the graphics card and
run off the motherboard video. Be sure to adjust your BIOS accordingly
Try moving around your System RAM DIMMs and/or try removing one
Try running an antivirus scan from CD, Diskette or Flash Drive if you have that capability. If needed, boot the recovery CD to a prompt first
Try booting with as minimal a system you can. Remember to disconnect optical and floppy drives. Also, try using a non-USB kybd and a
regular ball mouse that plugs into the PS\2 port in place of an optical USB type
If all this fails to identify or fix your problem, Try Running Chkdsk from the Windows XP\2000 Recovery Console.
If chkdsk fails to resolve the problem, at this point you may need to reinstall windows but first ...
Try checking out my Windows XP Boot Issues article to find out how to fix mup.sys hangs by manually restoring your registry. The process
I detail is reversible if you back up the original hives first as instructed so it is worth a try ;) The fundementals of this procedure should also work
on Windows 2000.I can say from experience that this absolutely fixes the windows XP mup.sys boot hang when the cause is a corrupted registry!
The motherboard my have failed or was damaged in a particular way such as from an excessive power draw by USB devices or AGP graphics card.
One astute and observant reader provided this feedback:
"We actually found this problem (mup.sys hang) was bulging capacitors on the motherboard" - Thanks MOH for the much appreciated feedback!

What is MUP.SYS and what does it do?

As I stated above it is highly unlikely that mup.sys is causing your boot issue but maybe you're curious. Just what in the world is mup.sys anyway?
Glad you asked! MUP stands for Multiple Universal Naming Convention (UNC) Provider. uh huh. That clears it up doesn't it!

OK, How about this: mup.sys is a kernel mode network redirector used as part of the Windows Installable File System (IFS).
Still as clear as mud? Maybe if I give an example of what mup.sys does it will become more clear.

Basically MUP comes into play when an application is trying to perform an operation that involves a remote filesystem resource.
Mup.sys acts like a router that calls the correct networking method to provide communication to that particular type of resource.
 
For example, If you wanted to use notepad to read a remote file on machine "fileserver1" the path might look like this in UNC format:
\\fileserver1\user\joe\readthis.txt

Notepad calls the I/O Manager and requests access to and a read of the file using the UNC filespec
As this is a remote resource, the I/O Manager asks MUP to handle this and passes along the request
Mup.sys then resolves the UNC prefix and passes the request to the appropriate network provider such as LAN Manager (SMB),
Distributed file system (DFS), webDAV, NFS, appletalk, novell, etc.. to handle the request
The network provider then fulfills the request by accessing the remote resource and passing the data back to notepad via the I/O Manager

I have greatly simplified this process, especially in regard to the network provider functions in item 4 but I think you will get the idea.

So what does mup.sys have to do with a boot freeze or loop issue? Most likely, nothing. The only possibility that
might even be relevant is the instance where your system uses mapped network drives. This paragraph is just for
your edification and satisfy your curiousity on what mup.sys does.

As stated above, I have never seen a case where mup.sys is the actual cause of the black screen of death on boot issue.
It is just the last thing you see on a problematic safe mode boot, nothing more!

Here's a challange, If you ever experience a situation where mup.sys itself is the cause I'd be most grateful if you could let us
know by sending an Email or submitting a message from our Contact Us page. With your permission, we will post
that here for the benefit of all acknowledging you with full credit and\or a link to your article if you wish.

All the best to you. Happy and successful troubleshooting!

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