The newer computers send a message to the OS, telling it that the power button was pushed, and it is possible to override this, BUT, when you hold the power button down for 5 seconds (this time may differ on some systems) it bypasses the OS, and shuts the computer down.

Heh, mebe try disableing everything, and then unhook the power button inside your computer? But that would require that you open up your computer every time you want to shut down, thats kinda lame, but if you realy need to make shure its not shut down, be shure to melt the power cord into the computer, and glue the other end on a UPS backup, then, ontop of that, make shure their is no way for anyone to unplug the fuse, heh, if you did that, you may also want to check yourself into a mental clinic.