I fitted a new power supply, motherboard, chip and RAM to my son's pc in Dec 10. Last week it refused to boot up. The fan on the power supply kicks in but the pc doesn't boot and the hard drive does not make the right noises (or any noise for that matter) that you expect on boot/access.
I have tried unplugging any unnecessary devices, replacing the hard drive and using the onboard video but to no avail. I have also checked all connections.
Not sure if it is of any help but my son told me that prior to its complete demise, he had to try booting it up (pressing on button several times before it started.