A friend brought over a Compaq Presario x1000 with a temperamental power button (which took many many presses to turn on), so I agreed to have a look.
First thing I did was disassemble it, which I didn’t really document. I took the back (partially) off, which I don’t think helped – really, the bezel above the keyboard (where the power button is) is important, and a couple screws in the back to take the keyboard off are probably all you need. This part by far took the longest.
Next thing I did was verify it was in fact the power button at fault. Pressing it manually did the same as the plastic button above (which just pushed down on this) – which is to say, nothing. Using a probe to manually connect the pins instantly turned it on every time, and there was nothing visibly wrong with the solder connections, confirming it really was the switch that was defective.
I looked around for a similar button to use, or something I could jam in that would work, but didn’t really have anything suitable. I decided instead to re-purpose the mute button (hopefully he will at least change the power settings to put it in suspend, rather than turn off.. I probably should have suggested that in retrospect). It had a ribbon cable connecting it to the motherboard, and shared a common ground with the power button, so I just had to route it over to the power button. After finding the correct wire with an ohm meter, I pulled it out from the ribbon.
Finally, some soldering and that’s it. The mute button now functions as power, and the old power button does nothing. I haven’t yet heard how many times he’s accidentally turned off the system..