Ot - computer won't boot

[Phil]

Thank you for reading this and I know that this a vague  question but you may be able to give me a lead.
For the past  couple of days my desktop computer has been very slow to begin the boot process, perhaps 5 minutes or so. Now it won't boot all.
The disk activity led blinks at about a 3 second rate, the fans run, and speed up once the system warms, and I can measure 5 and 12 volts at the disk drive. There is no video from the on-board video chip (which is normal until the system boots up) and the system won't boot from the cd drive either. I've reseated various connecters and the ram board.
So, the power supply and disk drive are propably ok which leaves the mother board, ram and cpu but how I determine which one is at fault? I'm at least a 1000km from civilisation so I cannot speak to service personnel. 

Sent from Samsung tablet.
Regards,
Phil
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