In some MOSS environments you may encounter this error. The Event is noted on the event log as:

Event Type:    Failure Audit
Event Source:    MSSQLSERVER
Event Category:    (4)
Event ID:    18456
Date:        20/02/2008
Time:        12:43:00 p.m.
User:        NT AUTHORITY\SYSTEM
Computer:    MOSS-WLGWFE001
Description:
Login failed for user 'NT AUTHORITY\SYSTEM'. [CLIENT: <local machine>]

The reason for this as I found out is a redundant SQL job related to the SSP database. The SQL server had been used previously to install and test a MOSS pilot and the error was related to a SQL Server Agent Job that was running against a non existent database related to the SSP.

Checking the SQL Log showed me that it was trying to connect to a non existing database.

To get rid of the Error you can disable the Job via SQL Server Agent Jobs or simply delete it.