My boss went out to a client who have a Windows 3.1 machine from the early 90's running a cad/cam machine making parts.
Off the top of my head I'd say probably the HP 4050 printer in ICT - I was told it was a couple of years old when I started here in 2004!
I've still got a broken psion / acorn palmbook which my parents brought when i started secondary school in 1996.
The first portable computer with an ARM chip in it.
I wanted one of these so badly when they were new.
In 2009 I retired an HP LaserJet 2.
I might win this one, while the science tech was clearing the prep lab out she gave me a box with a zx81 spectrum which is sat on the desk at work.
For the curious it was released March 1981!
Oldest in ours I've found is an old Celeron laptop running W98. Booted it up out of interest...by the time it had finished booting 10 minutes later I had lost interest
My last school still had several Acorns setup ready for use (but they wernt) until 2006ish...couldn't understand why the teachers had let them sit in their classrooms for so long
Our oldest bit of tech is probably the old Plasma Screen in the dining hall (we had a pair one has died) or maybe the 22" CRT Info Screens (only 1 remaining out of three as one died and one had an accident during the major building works and never recovered.....) or maybe it could be the CAT5 cabling and some of the last manky 40 year old phone wiring floating around the site..... Desktop PC wise we have a few 3GHz P4s floating around running display screens and things
We still have an acorn