I am in a pity if my PC is dead or it can,t get steady at all.
Its Spec is:PIV 2.6 Ghz,Intel D865 GBF DDR 333 512 RAM,Samsung HDD 80 GB,USB mouse,PS/2 KB,Win XP PRofessional OS

It has reached gradually to a state of full black screen.Now I can,t even enter the BIOS too.First it was random restarts and some restarts reached only upto the mother board logo display and it freezed.The monitor and power supply & RAM are tested OK in another working PC.

I put it to a bare minimum of motherboard,CPU and RAM with Keyboard only connected but no avail.The KB LIGHTS are flashing ones the powre is switched on.The cpu fan is turning well.The monitor LED is blinking as if there are no signal to it.
I removed even the RAM stick and then there are 3 beeps.If the RAM is reinstated, there are no beeps.I found that there is no thermal grease above the processor.Could it cause such no display problem?The Board is directly connected to the metal stand-offs.Is there any need of insulating material to avoid contact withe the case?

I have little knowledge with PC troubleshooting.I can,t help suspecting anything to a MOBO short,onboard VIDEO failure,processor malfunction etc.
I cant just understand upto what part of BOOTING my PC is going faultless.Or is it not able to just move from the starting point with MAJOR components FRIED?

Thanks in advance for any help!The minimum should be
MBoard
1 stick of RAM
CPU and fan
Video Card (if no onboard video)
PSU
Keyboard and Mouse.

If there is no thermal paste you need to do that first before you proceed. You risk frying the CPU.
Travel to Arctic Silvers site for excellent instructions on this procedure.

After applying the compound make sure the heatsink/fan combo are seated firmly...


Re-boot and let us know the results.

p.s. Double check all your PSU connections as well.Thanks Patio

I connected all as suggested by you.Applied thermal paste.No better results.If I connect CD drive and insert a disk,it is being spun as if being read from it.The HDD if connected shows its LED active for sometime at random intervals.The key board lights are flashing at first but not lit thereafter.I am eager to hear that famous 'beeps' that a faulty PC make but there are none!!!

Any suggestions?This is a puzzler...it seems you have taken the proper diagnostic steps here so far...
2 things you could try that won't cost much:
1) Replace the CMOS battery on the MBoard ( i doubt it's this but try anyway)
2) Borrow a known good PSU of the same wattage or greater and swap it in there...i know you stated yours worked in another machine however this may not prove yours had sufficient power for things to operate properly...

Whose MBoard is this and how old is it ? ?

Last resort would be to remove it from the case and do a bench test. To do this place the MBoard on a rubber mat...or use cardboard and hook up only the components i listed above and see.

p.s. I have seen these symptoms because a CD/HDD screw had been loose and dropped under the MBoard causing an intermittent short. Not saying this is your scenario but just explaining the logic behind removing the MBoard from the case...