Event Details
Operating System->Microsoft Windows->Built-in logs->Windows 2000-2003->Security Log->Logon/Logoff->EventID 551 - User initiated logoff [Win 2003 / XP]
EventID 551 - User initiated logoff [Win 2003 / XP]
Indicates that a user that had logged on interactively or by terminal services has started the logoff process.

This event means that the system started erasing from memory user's primary access token, which contains the user's security information and allows access to objects. After this token is erased, the user cannot access resources such as files or registry keys. When the logoff process (erasure) is completed, a Security 538 message appears in Event Viewer.
If a Security 538 message does not appear shortly after the Security 551 message, a program or service might not be managing its access tokens correctly. Although the user cannot access objects, the program or service might have cached an access token and therefore retained the ability to access objects. 

Find more information about this event on ultimatewindowssecurity.com.

Corresponding events on other OS versions:

Windows 2008

Related events:

Under normal access token system management conditions (see above) this event should be followed by the following event:
        Event Type:     Success Audit
        Event Source:   Security
        Event Category: Logon/Logoff
        Event ID:       538
        Date:           10/26/2009
        Time:           07:31:44
        User:           NT AUTHORITY\SYSTEM
        Computer:       DC1
        User Logoff:
        User Name:	DC1$
        Domain:		RESEARCH
        Logon ID:		(0x0,0x60FA64)
        Logon Type:	3
Log Type: Windows Event Log
 Uniquely Identified By:
Log Name: Security
Filtering Field Equals to Value
OSVersion Windows 2003
Windows XP
Source Security
Category Logon/Logoff
EventId 551
Field Matching
FieldDescriptionStored inSample Value
DateTime Date/Time of event origination in GMT format. DateTime 10.10.2000 19:00:00
Source Name of an Application or System Service originating the event. Source Security
Type Warning, Information, Error, Success, Failure, etc. Type Success
User Domain\Account name of user/service/computer initiating event. User RESEARCH\Alebovsky
Computer Name of server workstation where event was logged. Computer DC1
EventID Numerical ID of event. Unique within one Event Source. EventId 576
Description The entire unparsed event message. Description Special privileges assigned to new logon.
Log Name The name of the event log (e.g. Application, Security, System, etc.) LogName Security
Category A name for a subclass of events within the same Event Source. Category Logon/Logoff
Domain Domain of the account for which logon is requested. InsertionString2 RESEARCH
User Name Account name of the user that started the logoff process InsertionString1 DC1$
Logon ID ID of the logon session of the user logging off. Useful for tracking other user activity within the same logon session. InsertionString3 (0x0,0x60FA64)
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