The PC showed nothing on the monitor screen. Mainboard doesn't even beep too. Everything light and power are working properly. I've tried to plug in another RAM but it still not working. Any body suggest me an idea how to solve this problem??? Which part is dead? The mainboard? The CPU or the VGA??

In general, remove or disconnect every piece of hardware that's not necessary for it to boot up--all the drives and cards, except video. Make sure the RAM and cpu are firmly inserted. You may need to verify their slots are free of dust. If the power supply has a 115/230 voltage switch, make sure it's in the correct position. If you have 2 video outputs, try both. What happened to it between the last time it worked OK and the first time it didn't?

My CPU is P4 530 3GHz, I always have it running at 3.68GHz, it works perfect with that speed. The default DDR2 bus speed is 533 MHz. When I increased the CPU bus speed from 200 to 245, the RAM bus speed turned to about 615 MHz. Everything OK with that. Yesterday I decreased the RAM bus speed to 400 MHz, and I increased the CPU to 266 so that the RAM bus speed reached to 534 MHz. At that time the CPU Speed is 3990 MHz. The system still boot, but it never reach to the Windows boot-logo. I turned it off and the PC didn't boot til now. It showed nothing on the monitor screen. I'm using liquid cooling system for the CPU and every HDD also have coolers for themselves. The RAMs also have their own heat-speader. The PSU is 680W. I think the CPU is gone but when I took it the Fry's store they said it works fine????? Maybe they've wrong. My mainboard support both DDR1 and DDR2 so I tried to replace the current DDR2 RAM by a DRR1 but it still not working.

Assuming you changed the clocking in cmos/bios setup, try clearing the cmos. There should be a 'clear cmos' jumper on the motherboard. Jump it to the clear postition, turn the PC on for a second or two, turn it off and put the jumper in the standby position. If any of the changes were made via motherboard jumpers, return them to their default settings.

I've cleared the cmos and even pulled out the battery. I didn't change any jumpers in the mainboard too.

It sounds like your last act of overclocking may have been too much. But if you've cleared the cmos then I wouldn't think anything was still overclocked. If you've had the cpu tested and they say it's good then it probably is. And I wouldn't think if it was overclocked too much that there would be any permanent damage. You may want to leave the battery out overnight in case the cmos didn't get cleared. Then verify the cpu, fan/heatsink and RAM are correctly and tightly installed. Disconnect the drives and remove the cards if you haven't tried that already.

Try this once..take the CPU out from the motherboard...then short the power-up pins on the motherboard(that is...the pins to which your power-on switch of CPU is connected) with the help of screwdriver. I learned this just yesterday....my pc was giving trouble and then it stopped booting..nothing on the display...fans running.I took it to the vendor...and he did this procedure...and the pc started booting again.Told that the BIOS hangs due to which this problem happens. So you can also try this and see i if it works for you....best of luck.