well this happen with me too when my ups was not working
when i press the UPS on button my pc boot itself (i dont need to press on button which is on cabinet)
I made a thread on it too asking how to change the setting ?
but some mates told me that either ur button is damaged or ur bios setting is changed....
but that was not the appropriate answer, so i re-seat my cmos battery and the problem gone
Now i have to press my cabinet on button to boot...

dont wry , its not a majot problem coz i run my pc in this mode 3 months without any problem