Some privileges, such as Act as Part of the Operating System and Debug Programs, are extremely potent. An account having the Act as Part of the Operating System privilege (also referred to as the Trusted Computing Base or TCB privilege) essentially behaves as a highly trusted system component. No privilege is more trusted than the TCB privilege, therefore the only account that has it by default is LocalSystem. An account having the Debug Programs privilege can attach to any process and view andadjust its memory.
When a user logs on to a Windows NT or Windows 2000 system, Windows authenticates the account and creates a token for that user. The token is applied to every process and every thread within each process that the user starts up. The token contains the user's SID, SIDs for each group the user belongs to, and privileges held by the user. Starting with Windows 2000, the token can also contain information about which SIDs and privileges are explicitly disabled, these are referred to as restricted tokens.