Started having some hang-ups - I figured that is due to broken packages but now the machine started making trouble booting. I have tried reset - plugging off the power completely (which helped once), but now it won't boot anymore. Leds for harddrive and power come on but there is no response not even splash from BIOS. I assume it must be on CPU, more or less. Memory will cause problems later during the boot, right? It could be graphic adapter as well, but I think with faulty adapter the boot will proceed? Of course it could be the whole mainboard too.

Any bright ideas how I should start solving the problem? I have some spares, somewhere I should have an old AMD64 CPU as well or an AGP slot graphic card, even memory chips...

A quick suggestion is to buy a post(Power On Self Test) card. If you enjoy fiddling, that's for you. It sticks on the bios test that is failing, and you can diagnose accordingly. Powering off is capable of blow things if something is drawing too much current, but let's presume your dodgy part has turned into a hard fail. I would pull all cards and disks except video, and try again. If it comes up saying "Insert a diskette into drive A:" the motherboard is probably OK. If you have bits, you can swap out power supply and video card. then you have eliminated all except m/b & cpu.

The one time I have seen this, I removed a small piece of fluff that had lodged on the edge connector of a memory module. All returned to normal!
Try cleaning the edge connectors of memory modules and reseating them.

Please supply some details on the hardware---and what OS you are using.

If you do not have the manual for your motherboard, then you can probably get one on the manufacturer's website. We need to know the LED codes to help diagnose the problem.

Also, do you get the same symptoms on every boot attempt?

When was the computer last cleaned? (Complete cleaning means to blow--or vacuum--out all the dust, and to disconnect and reconnect everything except the CPU. Be sure to use static protection procedures.)

I have it running again. Disconnected the hardrives and stuff one after another - after I exchanged CPU and graphic card it woke up again. I suppose it was the graphic adapter in the end that did the trick - once I have time I can try the CPU again as this one is AMD64 3000 and the other one is 4000 - then on the other hand the machine has more or less served already and I am planning on new one but I need to access the data for some time still...