SQL Server 2000: Tracking Down Source Of Failed Logins

I was checking the logs because of some failing backups and found literally hundreds of enties for failed logins for user sa. This is does not seem to be related to the failed backups, because even after fixing the problem (disk space), the entries continue to pile up.

Needless to say, the information provided in the log is a bit sparse: date and time, source (Login), message (Login failed for user 'sa'.). If that is actually enough to go on, then there must be some way of cross-referencing this information with that found somewhere else. Is there a 'somewhere else' or are there other things I need to do in order to track this down?

I don't suspect any malicious intent, but I'm not ruling it out. My main concern at this point is twofold: to gain a better understanding of system management; to identify and correct the failed login.

Any recommendations, whether tools, documentation, or otherwise are very much appreciated.

Ron.