Memory error

This is about a Pentium II 400MHZ machine running Windows 2000 Professional, 128MB RAM, upgraded from NT 4.0.

We have recently changed our domain, and for one user I am trying to give local administrator rights on her machine. When I click the add button to add the user from the domain I get the following error message:

Application popup: mmc.exe – Application Error : The instruction at “0x69923705” referenced memory at “0x00000000”. The memory could not be “read”.

It is listed as event ID 26.

I have tried SP1, which is the solution Microsoft gives on the generic error message, but it didn’t help.

Any ideas?