First on your suspected list should be your power supply unit of your system Warkel.
If it is a few years old, then it is best to try your system with a new power supply, or a spare one, or even borrow one for a hour or so to test with your system.
Having a faulty power supply, or one that is a few years old can often be the cause of blue screens of death.
So if you can do this to confirm it is your power supply at fault in the system it stop you from wasting money buying a new one first of all if not the cause.
Your next two culprits are your system memory, or the cpu.
With the memory run a Microsoft memory test program that you can download from the link bellow.
You will have to create a disk,burn to Cd-r or dvd-r the disk is boot able so set the dvd or cd drive of your system to the first boot able device in your bios of the motherboard if not set already.
https://technet.microsoft.com/en-us/magazine/2008.09.utilityspotlight.aspx
If the memory test pass without errors, then it`s not the memory at fault.
Next you want to be looking at the cpu, and or checking the temperatures of it.
If they seem quiet high, then you should check that the cpu cooler on top of the cpu is in firm contact with the cpu die.
You should also visually check the state of the cpu cooling solution, if it has been a few years then check it for a thick matte of dust or clogging between the fan on the cooler and the cooling fins of the cpu aluminium cooling block.
If clogged take of the fan on the cpu cooler, and with a clean paint brush, or air can remove all of the dust.
A overheating cpu can cause Bsod or system shut downs.
If you have a Pci-e based graphics card in your system also check the cooling solution for dust build up, if the card has a blower type fan on it, or what we call a paddle wheel type fan. check for a build up of dust that will build up on the edge of the cooling fins of the cooler just behind where the fan blows cold air through it.
And overheating graphics card will also cause BSOD and also a condition where your monitor screen will go a solid black color and the system becomes unresponsive, or will randomly reset.
Just go through the check list one by one.
How old is the system you have BTW ?