|"I haven't yet tried the monitor on another computer. Because I don't have any spare part."|
Surely you know someone who has a computer you could connect your monitor to. For that matter, you could take your computer case to where that person is and try their monitor with your computer.
If your LCD monitor does not work on the other computer even while booting before Windows loads, it is common for the voltage inverter and/or the backlight to fail after an LCD monitor has been used for some time.
For some info about replacing a voltage inverter or backlight, and what some typical symptoms are, see response 5 in this:
"But I am sure that the PS and the CPU are working fine. I looked inside the computer removing the cover, the CPU fun was spinning. "
The cpu fan spinning is a good sign and indicates the cpu has probably not overheated and damaged itself, assuming the heatsink was properly installed and there is adequate thermal grease or a thermal pad there - if you have not re-installed the cpu heatsink or have not had to replace a dead cpu fan it is probably okay.
You can't tell if the PS is working properly by just looking at it and seeing that the fan for it is spinning. It is common for a PS to partially fail - it's fan may spin, other fans connected to the mboard may spin, hard drives may spin, but the computer may not boot any further.
A failing PS can produce too much voltage and damage components connected to it and the mboard - including the video.
"I even tried replacing the CMOS Battery and reseting the ROM, but none of these helped."
That almost always doesn't help for a problem like yours.
"When I insert an OS CD in the CD-ROM it tries to boot from the CD.
When I reboot the computer the keyboard lights lit (detects the keyboards), it detects the hardisk (I can hear the spinning sound)."
That's encouraging, but it doesn't necessarily mean the computer is working properly other than the video.
If the hard drive is accessed repeatedly after the initial part of the boot, seeming like it is actually loading Windows like it did before your video problem appeared, then you can be pretty sure that either your monitor or its connection to the computer is not working, or there is something wrong with the video card or its connection in it's slot is poor - the latter is very unlikely if you have not been fiddling with the video card physically.
Is this computer and everything connected to it protected from power spikes and surges? Whether or not that's the case, have you experienced any power outages or nearby lightning since the video last worked properly?
"At first I thought the problem was with the video card so I checked it by adding a PCI video card which was working in another computer, but there was no change..."
Sometimes people say video card when what they actually have is onboard video.
Does your computer have onboard video (a 15 pin hole D shaped port directly connected to the mboard), or does it have an AGP video card in a slot, or does it have onboard video and an AGP slot with no video card in it?
If you have onboard video and a card in an AGP slot, you will get no video if you connect to the onboard video port because the onboard video is auto disabled when there is a card in the AGP slot.
If you have onboard video and a card in an AGP slot, if you did remove the AGP card when you tried the PCI card, or if you have onboard video and an AGP slot but no card in the slot, on many mboards the PCI card will not work either because the bios expects an AGP card to be in a slot in order to disable the onboard video, the onboard video is not disabled, and the onboard video and PCI video card clash because they are trying to both use computer resources at the same time - if you get a display at all on the PCI card, it is garbled.