computer won't start

[iamtheblues]

I wonder if anyone can tell me what the solution to this might be. I appeciate that it's a physical problem rather than a virtual one relating to software, but I wanted to see if there was an alternative to replacing everything between the on/off switch, and the socket with the pins attached to the mainboard.
When turning off Windows it will not boot up again. When restarting Windows, it does (used to) reboot. I think the problem is the cable between the on/off switch and the mainboard. After the the last time I switched Windows off completely, it will not restart.
I think, but I'm not sure, that the black white cable marked POWER SW is the faulty one. Jiggling it's position on the pins on the mainboard would occasionally do the trick, but it seem to have failed completely now.
The fans still work when the power cable is connected to the electricity supply.
I think that the power light on the front used to glow steadily, but now it flickers rapidly.
Any suggestions appreciated.
The attached diagram was made with Xara, and was intended for the man in the local computer shop, hence the spanish. But once the pc enters his shop, he becomes costive about letting it back out again. So I'm reluctant to let him have it. Last time it was nearly 8 weeks.
Saludos,
Bob.