Anoymous login in windows 2000

Hi ,
I have win2k pro with SP2 ,I have only 4 users created which includes the guest and administrator ,and one with user level access and one with administrative previlege user ..So totally i have only 4 users in this …
Recently when i saw security log i saw that some user anonymoys logon user logged in ..
below is the descritpion
Event Type: Success Audit
Event Source: Security
Event Category: Privilege Use
Event ID: 576
Date: 5/28/2003
Time: 11:50:33 AM
User: NT AUTHORITY\ANONYMOUS LOGON
Computer: SMC2176
Description:
Special privileges assigned to new logon:
User Name:
Domain:
Logon ID: (0x0,0x71FF)
Assigned: SeChangeNotifyPrivilege
I have no idea who is this is User ,is it normal ,is it a securitybreach ..
Please help me
regards
Anantha krishnan