The powering on issue when plugged in maybe it was setup this way in BIOS.
As it is a PIII wich realy is not an issue as I have a PIII running with XP pro.Is there any way you can test hardware from this old computer?IE,hard drive by fitting as a slave in another pc?
What colour is the light on mainboard?
Do you get any beeps on startup or during the running time?If yes how many?
Has it got
RAM/Memory fitted in there?Do you have any old
RAM you can test in there?
You can try disconnecting all unesential hardware,say hard drive,CD/DVD rom.Test any
RAM stick by stick as in individual one at a time.