Logon Failure: Reason: Unknown user name or bad password User Name: webmaster

Could somebody please tell me where the user "webmaster" tried to log on to my server?  It appears that it happened from on my server.  We don't even have a userid called webmaster.  We keep port 80 closed and only have 443 open.  Did a program that sniffs 443 stumble across our private website from the outside?  Note that this happend exactly at 6:00pm.  We are running Windows Server 2003 SBS and Trend Micro Enterprise Antivirus.  Could it be one of my serivces (like the Trend Micro) trying to log in?

Source Event ID Last Occurrence Total Occurrences
  Security 529 2/27/2007 6:00 PM 1
Logon Failure:
  Reason: Unknown user name or bad password
  User Name: webmaster
  Domain:  
  Logon Type: 3
  Logon Process: Advapi
  Authentication Package: MICROSOFT_AUTHENTICATION_PACKAGE_V1_0
  Workstation Name: AJAX-SVR-2
  Caller User Name: AJAX-SVR-2$
  Caller Domain: AJAX
  Caller Logon ID: (0x0,0x3E7)
  Caller Process ID: 1528
  Transited Services: -
  Source Network Address: -
  Source Port: -