MMC.exe

research is gewd:

It is caused by a running program/application which has a bug referencing the
content of a pointer (an address), but the pointer is NULL thus no content
can be read. You should try to identify the offending program/application.
And ususally when this message appears it also indicates which
program/application is attempting to read this memory location.

To me, this sounds like a corruption of either Contorl.exe (the application that is the Control Panel) or, more likely, a corruption of the .CPL file that runs Users and Passwords.
Try this:
From the Command Prompt, type in:
control userpasswords

On a working 2000 system, that command line would open the Users and Passwords applet. If we are lucky, that will work for you.

Now, if it doesn't work, then I would get your Windows 2000 CD and extract Control.exe and every .CPL file on there, and put them onto your 2000 system. After that, run SP2 again.
From looking at my system's own $NtServicePackUninstall$ directory (the directory the SP install makes when you select for it to copy the old files in the event you want to remove SP2), Control.exe is NOT in there. Also, only a few .CPL files are in there.
This tells me that Control.exe and most of the .CPL Control Panel applet files are not replaced by SP2. This might explain why loading SP2 did not fix this.
Now, if you are wondering which .CPL file specifically runs Users and Passwords, I unfortunately can't tell you. I can't find it. I thought it might be Main.cpl (which runs Printers, Fonts, Keyboard & Mouse, using the appropriate switch), but that is not the case here. So, that is why I say get all of the .CPL files from the CD, copy them to the 2000 machine you are having problems with, then put on SP2 afterwards to update the few it does change.

Here is a Technet article about all of this:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;Q180025
(please remove any spaces)

Hope this helps.

One of the additional tools that comes with the windows2000 and server versions ([cdrom]:\support\tools?) is dependancy walker. While it may not give definitive sense, it may provide some clues.

First, open dependancy walker (start -->run --> depends) then open the mmc application file. Next go to profile and start profiling, add the users snap-in and try adding your users, note what files/dll's are being accessed, what errors are being reported by the application and this may or may not give you a clue as to what is failing.

hope this helps