"I swapped the BIOS chip"-what about the CMOS battery-maybe that needs to be replaced. Also, I would check the heat sink and fan for the cpu before concluding that it's a bad Mboard.

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CMOS batttery won't stop the board from POSTing.

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You are shotgunning. Wildly replacing parts. Changing things that are completely irrelevant. Shotgunning can even exponentially complicate your problem. Locating what is defective is easy and takes less time if you disconnect nothing. For example, did you know a power supply 'system' includes more than just a power supply? Is the power supply controller working? Do you just keep buying parts until over half the computer has been replaced? Those who can better answer your question are mute because you have provided no useful facts. Those necessary numbers can be obtained in but 30 seconds with a 3.5 digit multimeter. Measure voltages on power wires where they enter the nylon connector on the motherboard. With system off (but connected to AC mains), measure the purple, green, and gray wires. Purple must always report a voltage above 4.87. Then measure both the green and gray wires when the power switch is pressed. Green wire should drop from well above 2.0 volts to near zero (well below 0.7 volts). Gray wire should rise to well above 2.4 volts within a second. Post these numbers to have a useful reply. Also useful are voltages on any one of orange, red, and yellow wires as the power switch is pressed. Also post those numbers so that the few who actually have electrical knowledge can eliminate also every 'suspect'. Almost every 'try this or try that' recommendation will probably be eliminated immediately and without doubt once you have provided those numbers. The meter is only for layman and teenagers which is why it is sold in Kmart, Radio Shack, Lowes, or any hardware store. Even available in Wal-Mar for less than $18. Spend near zero dollars to have an immediately answer. Or spend massively replacing perfectly good parts while exponentially complicating the problem. Your choice.

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Okay, here comes the numbers... At the time I took those measurements the PSU had the load of one CPU, two pcs of RAM, one fan and a GPU. System OFF measurements: Purple: 5.06V Green: 5.09V Gray: ~0V System ON measurements: Purle: 5.06V Green: 0.07V Gray: 5.17V Red: 5.17V Yellow: 12.22V Orange: 3.36V MOLEX with system turned ON: Red: 5.18V Yellow: 12.23V If any other measurements need to be taken please let me know...

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Without the model of the PSU we are guessing as to what you might have. Many recent PSUs have MULTIPLE 12v rails. You are only measuring one. Also, I am not sure measuring the voltage by itself is sufficient. Why would you think the BIOS chip was bad? You may have damaged the chips or the board by pulling the chips. Do you have a chip puller?

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Those numbers suggest the power supply system is OK. In fact, that the power supply probably has much more power than the computer really needed. Better would be numbers from the red, orange, and yellow wires when the system is accessing (multitasking to) everything simultaneously. IOW play complex video graphics (ie a movie) while downloading from the internet, while searching the hard drive, while playing loud music, while reading a CD ... Now confirm those voltages remain above 3.3, 5, and 12 (as you have seen). But that means you can boot the computer. While you have the meter, you can check the CMOS battery. Simply measure that (maybe 3.0 volt) battery. If 2.8 volts, then plan on replacing it in the next 6 months. If below 2.6 volts, replace it now. Sometimes a low battery can make power up problems. But if voltage is above 2.8 volts, battery is definitely not a problem. Better is to know what was defective before seeing the problem solved. Measure the battery first rather than just replacing it. Power supply controller has a protective lockout function. If that function activates, then you will no longer see voltages change when the switch is pressed. That lockout function clears when removing and restoring the power cord. Also useful would be those voltages when the system will not restart. Normally, only a few hardware items can crash a multitasking OS. These include video controller, sound card, CPU, only some memory, a few motherboard functions and the power supply. However, only the power 'system' can keep power from also being restored. You are seeing power coming on just fine. Obviously, when you pressed the power switch and read those voltages, the disk drive, etc all powered up. So move on to other functions. Does the system always do nothing when the power switch is pressed? A first function executed on power up is the BIOS. With all memory modules removed, the BIOS will see no memory and beep the speaker. If it does not even do that, then the CPU and related motherboard functions are suspect. BIOS does not fail. Typically, CPUs don't fail. But a CPU power supply (those capacitors and voltage regulators that surround the CPU) can fail. Sometimes those capacitors might be seen budging. But your sudden failure would be more typical of semiconductor failure. CPU power supply semiconductor replacement means a new motherboard. Of course, if the CPU does beep that speaker, then the above paragraph is irrelevant. But your numbers say the power supply system is perfectly good - no power supply swapping will be useful. Of the few items that can crash an OS, even less would cause the computer to not boot. For example, memory and disk drive are completely and obviously not on the suspect list. The CPU and its support functions (ie its power supply) are likely suspects. If the CPU does not beep the speaker, then the CPU is not even executing its very first instructions - the BIOS.

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