In general HKEY_USERS\.DEFAULT in 2000 and especially XP references settings before any user logs in. For example, if you change the font size in HKEY_USERS\.DEFAULT it doesn't apply to every user, but it will apply to the classic logon window. Basically, every user has what is called a registry hive, stored in %ProfilesDir%\%Username%\ntuser.dat. This is where all their registry settings are stored. When a user logs on their registry hive gets loaded into HKEY_CURRENT_USER, and it is unloaded when the user logs out. Default user settings are stored in the Default User's registry hive, i.e., %DefaultUserDir%\ntuser.dat. Like schalti said, HKEY_USERS references all the registry hives of users who have logged in at some point by using their SID. That's the {S-1238-123987132987-blahblah} stuff you see under there. Registry tweaks applied to HKEY_CURRENT_USER will only affect the currently active profile. Most of the time this means the user that is currently logged in. However, during the installation process the Default User's hive is loaded into HKEY_CURRENT_USER, which is why during unattended setup you want to import user configuration stuff to HKEY_CURRENT_USER. In XP regedit can now load and unload hives (you used to have to use regedit32). Also, there are command-line utilities installed by default in XP that were once part of the resource kit in 2k that allow you to mount, unmount, and search registry hives, like reg and regfind.