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Computer turning off and on, on its own....

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As a 30+ yr systems technician I find this problem and the sugestions very interesting.
My first suggestion is to go back to square one in your troubleshooting flow chart.
So, IS IT ENVIRONMENTAL? In other words does it have to do with the placement of the tower where slight changes in the position caused by walking past it in one direction is causing the reboot?
To test that, remove the computer from the current location. Take it to a table in your garage or your kitchen and set it up there. Does the system still reboot randomly? YES/NO
If it does the problem is following the computer and not the location.
There are many good suggestions made concerning your power connections and some not so good about other cables etc. These other cables should not interfere in any way. Loose or worn out outlets on a power strip could cause this to occur. Since it can occur when the MOBO is still posting, I would rule out the OS and virus related issues since a system has not even booted yet.
I do question the power supply and the fact that you have 4 HDDs in this system. A 400w supply may not be enough, but then again, that doesn't explain why it happens walking past it in only one direction. Or why you can sit in front of it for long periods with no problems.
My first take is that something with the mobo is flexing just enough as you walk past.
Try putting the tower on it's side, which will change or eliminate the way the case is able to flex. Modern cases are built with much thinner materials.
Check that all the screws that hold the mobo down are properly tightened. Most of those connections are grounded to the chassis and if even one relaxes when flexed it could cause this sort of problem.
Two things you can try regarding the power supply unit.
First, try disconnecting HDDs that are not required or even all of them and the CD/DVD, floppy etc. Anything that has a power cord going to it. Be sure to pull the data cables loose too.
If the power supply is being overworked, even the slightest amount of stray inductance can cause it to 'crowbar' the system and cause a reboot.
Stray inductance could be introduced simply by walking past the computer. Also when a tower is in an upright position there is a double layer of the case protecting the mobo on just one side and not the other.
Second thing might be to try swapping the power supply out with your other computer.