I started having the same problem with my XPST a couple of years ago. You may have to press the start button and hold in several times to get it to boot. Pressing once and holding it in won't work (unless you get lucky the first time). Keep trying the press and press again until it starts. If it does, I would set the BIOS settings (AC power) from 'last state' to 'on' and use a power strip to power on the unit.
I should note that now that you have 'on' selected in your BIOS, do not simply shutdown the computer with the power strip. Use the Start>Shutdown>Shutdown button to shut down your system. When it has finally shut everything down (usually with a thump), then power off with the power strip switch. To turn on, simply use the power strip switch. Been doing this for over two years with no ill effects, including the CMOS battery staying charged.