Troubleshooting a faulty computer

I helped L's mum fix her faulty computer. Took a bit of time and imagination but I'm very satisfied I managed to fix it. Here's a walkthrough of the thought/ troubleshooting process.

When a computer doesn't power up, what could be wrong?

1. Power supply faulty. Check power supply unit. Remove cover from the computer case, remove power supply, and inspect for any burnt/black/melted components. The power unit in L's computer, without much doubt, had melted down. Could see a couple of components which looked more like goo than electrical components.

So I happily deduced the power supply was faulty. Went down to Sim Lim the next day and bought a new power supply unit. After installing the unit correctly, the computer still doesn't power up.

What else could be wrong?

2. Never on power switches. Duh.. Of course I checked before step one. But I checked them again this time. Nope, all switches are on.

3. Faulty power cable. Unfortunately at L's place, they only had one computer power cable, and only one component using it (the computer). Bo bian, cannot check (unless I stick two metal objects into the cable with my hands and seeing if I get an electric shock)

4. Blown fuse in the cable. The fuse showed traces of brown, might have blown. Replaced fuse with another one, and the computer still didn't start.

What else could be wrong?
At this point I nearly gave up. Decided to bring in an old computer to check another time...

The old computer could start. Switched the cable to L's com. Hmm. Didn't start. Given the power supply was new, and the cable obviously works, I can only deduce one other fault...

5. The power switch on the computer case was not working. It felt kinda "soft" to be honest, and didn't have the "click" when you press it. I decided to dismantle the switch from the case. Which involved 4 screws, a lot of prying and some guts to potentially crack/ damage the case. I didn't have a lot of the latter, so I thought I'd shift all the components from L's computer into the old case I brought.

After some time unassembling the components, I realised that the components from L's computer didn't fit into the old case! It wasn't compatible because of the power/reset/hd light connections.

Seeing no other way out, I decided to gather some guts and pry the switch out of the case. After I did that, I was more convinced the switch was the problem, because the switch had lost its ability to spring back out once it was depressed. I reconnected all the components and tested the switch, this time by pulling the switch back out instead of pressing it in. And it worked!

Once I had identified the problem, things became simpler. I pressed and pulled on the switch like 1000 times to loosen it, added some veggie oil as lubricant so it will spring back out, and fixed it back into the case. Computer fixed =)

Its such an innocuous component, the stupid switch... I wish I had identified that it was the problem sooner.

* update: it seems the computer has failed again. This time I think there's something wrong with either the graphics card or the graphics port. *

** update 2: I think I've fixed it again. At least, I'm using it now. I was wrong, there's nothing wrong with the graphics card/port. Apparently there's something wrong with the sound card, because once I unplugged the sound card it works fine. I've re-installed some drivers for the on-board sound to work. **