Missing Log Events

Need an explanation…
After reviewing the Event Logs (all 3) on a 2k client I found, the logs started 12/4/200 and end 11/30/2002..
During the review I noticed that the events from 6/18/2002 thru 8/24/2002 are missing. The last 6/18/2002 event in the Security Log is (515) System Event and the next entry 8/24/2002 is (515) System Event. After checking the other 2 logs I find that the same dates are missing.
How can this happen? Is third party software available to “EDIT” security logs. To add confussion, I checked and found multiable files that were updated during those missing dates.
The Aduits are set to record ALL Success and Failures.