My friend's pc suddenly went off one day, I got him to check/swap power cables as the monitor still had power but no, the pc tower still dead as a dodo, not a beep, whir, no lights, absolutely nothing.

I assumed the pc power supply had gone so got him a new PSU, but now the machine is still dead, the only sound from the new PSU fan, no other lights or sound.

I assumed the motherboard was fried so we install a new one, the exact same model as the old one. The machine has lots of noise, cpu fan on, lights on, usb ports front and back powered up, cd-rom opens, but the machine still doesn't boot up, monitor says "no signal".

I assume the old PCI-E graphics card has gone so we replace that and guess what!?! yep, it still says "no signal" on the monitor. The monitor does work as we tested with a laptop.

So my options now:
1. Replace CPU
2. Replace Harddrive.

If the cpu was completely gone, would the cpu fan still come on as it works at the moment.

If the HD has had some horrendous failure should the machine still attempt to start up, ie get as far as the bios screen?

I'm not sure what to do next, anybody got any ideas, all feedback most welcome!

Thanks!