At that moment you need to physically switch off your PC. The old PCs did not have automatic switch-off, as this is a facility in the Power Supply, combined with settings in your Power Saving setup.
To have the Auto Shutdown work, rightclick anywhere on the desktop, select Properties/ Screen Saver/Power/ and then Advanced or APM, (can't remember W98) and set it up.
This only works if that PC has a 'switching' PSU.