By switch off, I mean that initially, it switched on for 3-4 seconds, before returning to the point where I would have pressed the "on" button, except that the computer tried to start itself again. After this repeating several times, I then switched the power off at the mains.
After switching the mains power on again, and pressing the Go button, it again fired up a couple of times, eventually getting as far as opening Windows, before immediately switching off, and thereafter reverting to the DOS screen. No "system disk not found" message, simply the various options- "Start Normally", "Repair" , "Start in Safe Mode", etc. on a black DOS screen.
Hope that this clarifies things!
Thx