Thanks for your answer. I know about windows repair. However, the machine is running Windows XP home addition and does not give me an option to repair windows. The computer had two partitions C and D. The funny thing is that if I say install operating on D and then cancel installation, the computer boots with no problem. Then after a week it fails to boot again with no errors and black screen. I gave up fighting the problem and fixed the computer by re-installing an old Image of the operating system I made by Acronis software when computer was OK.