Computer freezes and won\'t boot

i would try and bare bone the system.one stick of ram and tsee if you get it to post to where it says no boot device found. if it still hangs at the same spot see if you can borror another video card or if the unit has onboard video to try that. if it hangs with new video card and one dimm it could be the mb or cpu at that point.

The most common cause of a computer failing like this is a faulty power supply. Measuring the voltages is no indication that the supply is good. The only reliable test a home user can do is to substitute the power supply.

Thanks for your input. With all due respect, the only other thing which can be a problem with the power supply is bad filter capacitors, which would create a ripple in the voltage. This can be analyzed simply with an oscilloscope, which I have. Is this what you are referring to?

I am a home user and an electronics engineer with 30 plus years in R&D and designing large control systems for industrial equipment. I do not want to purchase another power supply in order to swap and test, only to find out that this is not the problem.

I am also an electronic engineer and I also repair power supplies. The cause of a power supply failing is normally as you said faulty capacitors, you didn't say that you checked the output of the power supply with a oscilloscope, which is a device most people don't normally have. Even so if you can, try another power supply.
Assuming it is not the power supply then there are only two likely candidates left, the memory and the motherboard. Try downloading and running memtest86 and run for at lest three passes, try running with only one stick of memory. If the memory passes then the motherboard is the most likely suspect, try connecting the hard drive to a different SATA port.
It is difficult to diagnose a problem like this without spares and I wish you the best of luck, let me know here what the problem finally was.