What you are doing with powerstrip is editing the actual monitor driver. Your regular monitor is probably a plug and play monitor of some sort, and when the pc boots up it will load that monitor driver. When you boot up the PC with the arcade monitor attached it is not plug and play and therefore windows loads the Windows default monitor. That is the driver that you need to add the resolutions to in order for the PC to boot up displaying the right freq. Here is how:
First, uninstall powerstrip.
Reset the PC with >NO< monitor attached to it. This will load the Windows Default Monitor driver.
Install powerstrip again.
When it turns on, it will say "New hardware detected with XXXX video card on Windows Default Monitor".
Go to powerstrip, add all of your resolutions. Restart the PC again with NO monitor attached.
Now plug in the PC monitor again, switch the resolution to the arcade res. Plug in arcade monitor.
Reboot the PC and now Windows should boot in 15 kHz.
Anytime you want the PC to boot up in 31 kHz mode just turn it on with the PC monitor attached.
Let me know if this works!