In Windows NT 4.0, file association is managed globally, and stored in HKEY_CLASSES_ROOT, an alias for HKEY_LOCAL_MACHINE\SOFTWARE\Classes.
If you run Windows NT 4.0 Terminal Server Edition, the Peruser tool allows you to configure file type associations on a per-user basis.
Windows 2000 still manages global associations at HKEY_LOCAL_MACHINE\SOFTWARE\Classes, but has added the ability to have user specific associations at HKEY_CURRENT_USER\SOFTWARE\Classes. In the case of duplication, these user specific associations take precedence.
The Windows Explorer user interface allows Administrators and Power Users to manage the global settings. If you wish ordinary users to manage global associations, use Regedt32 to navigate to (or Add Key):
Edit or Add Value name NoFileAssociate, a type REG_DWORD. A data value of 0 enables the buttons on the Windows Explorer / Tools / Folder Options ... / File Types tab. A data value of 1 disables them.
NOTE: There is no Group Policy for this entry, unless you create a custom .ADM template.
NOTE: HKEY_CURRENT_USER can contain a user specific NoFileAssociate.
There is currently no GUI method of managing the per-user associations, you must edit the registry.
NOTE: I added a custom association for my account and it became effective immediately.