Microsoft KB: link
SID: S-1-5-18
Name: Local System Description: A service account that
is used by the operating system.SID: S-1-5-19
Name: NT Authority
Description: Local ServiceSID: S-1-5-20
Name: NT Authority
Description: Network Service
SID: S-1-5-21-3580036505-2828148361-1391708163-1001
* A subkey named for the Security Identifier (SID) of the current local user. This subkey contains the current user's profile. The data in HKEY_USERS\SID also appears in HKEY_CURRENT_USER.SID: S-1-5-21-3580036505-2828148361-1391708163-1001_Classes
* A subkey named for the Security Identifier (SID) of the current local user with the _Classes suffix. This subkey contains the current user's Classes. The data in HKEY_USERS\SID_Classes is also contained in HKEY_CLASSES_ROOT.