You need to do a repair install of XP. You will need an XP disk. See the instructions posted in this forum, forth post from the top. Follow the instructions carefully. The repair does not erase your files.
Does the Bios even come up? Or is it a black screen and error beeps when you hit the power button?
If it comes up but just goes nowhere it's windows. If you're getting nothing when you hit power but an error beep it's a hardware issue. depending on the motherboard you may be able to correlate the beep codes and it'll tell you what piece of hardware is wrong. For starters you could make sure everything inside the case is plugged in correctly and nothing has became loose.
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.