Disconnect the hard drive, all CD / DVD drives, floppy drives and any card readers plus adaptor cards and so on.
You are aiming to have graphics card, CPU and RAM only installed on the motherboard, now with it connected to the monitor switch it on and observe the screen, does the system now show the POST screen?
If it does power down connect one device and power on again, if the system still POSTs repeat the steps until it fails and the last device added should be the one causing the problem.
If the system does not POST power off, ensure that the graphics card, CPU and RAM are all seated correctly and try again, if it still fails try resetting the BIOS to defaults, if that fails clear the CMOS with the relevant jumper and try again.
Phono I have tried with just those components, I think the screen stayed blank - but I will try again tomorrow evening. What exactly does POST mean please, I understand it may be part of the boot up procedure, but I do not know exactly what happens ?
POST is Power On Self Test, with those basic components installed, and provided none of them are faulty of course, the system should carry out the POST and you should see a black screen with white text, albeit the text may scroll or change rapidly.
If you are not seeing the POST screen then one or more of the motherboard, CPU, graphics card or RAM is/are defective or not properly seated.
I should have asked earlier but does the system issue any sort of beeps when you switch it on?
This could be a short single beep, one short and three long beeps or various other combinations, if it does this could be of great help in diagnosing the problem, who is the maker of the mobo and what is the model number?