Like David, I change computers to often... the oldest file I found on my laptop was only from 2003 (which still predates this laptop by 4 years). Not surprisingly, it's a IM transcript of me helping someone fix a netscape email configuration problem. I'm so glad gmail and google apps have made that a thing of the past. I'm not even really sure why I saved it.
Jan 31 2003 /Users/jczebota/Documents/im_netscape_mail.htm
The oldest file I could find is a file from 2000 on a flash disk in which I was tracking and summarizing $185k worth of subcontractor work that I was managing as part of a New Building Steering Committee for my church. I remember that time with fondness as it was quite a bit of responsibility for being just days after I turned 19, and I excelled at that.
Jul 25 2000 /Volumes/ONEISHY2/jehiah/NCCSumary.doc
And something that half-counts is some slightly older email records that i still have from Tue, 31 Aug 1999.