I'm betting that you have a system policy that changes the permissions on the %systemroot% folders to prevent ordinary users from making changes to the system files. It's possible that one of these files is required by a user in Windows 2000 and that is causing the c0000021a bug check.
Look over your configuration files, logon scripts and system policies to see if you are making any permission changes. If nothing looks out of whack, the next step is to create a group that is exempt from system policies and logon scripts and see if it has the same problem. If so, then you might have deeper issues.
Just as a thought, do you have a Default User profile in your Netlogon share? Some shops use this to provide a standard initial profile to new users on their desktops. There might be a setting in this default profile that is causing the W2K machines to cough up furballs.
This was first published in February 2002