Most Ancient?
I've been using this as almost my only computer for many years - a Compaq P-III, 866 MHz, maxed out to a now-pathetic 512 MB, running Windows 2000. It's from around 2000, and originally came with the beloved (?) Windows Milennium. I think it's also run W98 and XP at one time or another.
You'll laugh when I tell you the replacement I got for it that I haven't set up yet - a 2003 HP/Compaq office model with a 3.4 GHz Pentium 4, running XP. But for what I do with a computer, this "new" one will be quite a speed demon. It will be nice to watch videos play smoothly, and compress video files in minutes rather than hours. But I'll mostly be doing web browsing and email. And Freecell.