The PC of a friend usually switches on OK, but occasionally when she presses the power button the button lights up as if the PC was on but nothing else starts (not even the fan or the blue lights on the front!). When this happens she has to switch the mains off and on then try switching the PC on again - though this doesn't always work.

She's tried ringing her tech support, who claim to think it's a software issue. But I think that since it can happen having had the mains power off, it's got to be something to do with the hardware - perhaps the switch or the power supply.

So am I right that it's definitely hardware (if so, she can get it looked at for free) and has anyone got any idea what the cause might be?

Hi JDW, welcome to PCHF!

Not unless they are talking about the BIOS which *could* be considered as software, however this would have been the first time I have found the BIOS to cause this particular issue.

9/10 times, this particular problem is caused by a faulty or failing power supply. You can rule out both the motherboard and the BIOS, next time this happens, instead of removing the power cable, unplug the two molex connectors that plug from the power supply to the motherboard, and then plug them back in again. If the PC doesn't start, then it's a power supply issue. Also double check that there is enough power coming from the power supply from here: eXtreme Power Supply Calculator v2.5 (click 'go lite', and add up to 15% for Capacitor Aging).

EDIT: Marked as answered.