My PC has finally recovered! It has been a long way. I didn't know what was wrong, but today I bought a new RAM and set it. Then it worked!
The most difficult part in fixing a computer is that you cannot easily find the cause of disorder. In my case, I was suspicious of the motherboard, the hard disk drive, CD-ROM drive, and so on. If I were an expert, then it would have been much easier to find what was the real problem. But I am not. I could not tell which one of them was a nuisance while the other were all right. I was lucky because my guess was right, and just buying RAM solved my problem. Otherwise, I would have had to buy several other things to check which does work and which doesn't.
I recommend you to buy a ready-made PC from a shop. Never try to build it for yourself!