Sounds to me as if your Windows installation does not have the correct drivers for, or cannot for some other reaosn control the APM chips on your motherboard. If APM is enabled in the BIOS, Windows should use the BIOS APM. If it's disabled in the BIOS, Windows uses it's own.
To be honest the APM in W9x isn't much cop. The standby function is notoriously unreliable, sometimes not working at all and othertimes its puts your putah to sleep with lethal injection where your reset button doesn't even work.
Unless you need specific standby features, I'd use the Monitor and HDD settings withing the screensaver control panel. These are the bits that use the most power on your PC.
Aq