Hi hi!
Simon is right with the check there - remember your power lead has a fuse in it. If its the wrong type of fuse, then a power surge could get through (instead of blowing the fuse) and blow your PSU.
But, to give you an idea of how sensitive things are, ANY device, even a mouse, can cause power issues that make voltages unsteady on the mainboard, and of course, therefore the main power PSU.
Are you attaching ANYTHING to the DELL that didn't come with it? You have had the tower replaced twice, so I would doubt that it is anything inside that came with the dell. But, nothing is in stone which is why you get warranties. It is possible that you got two duff dells' (they received a duff shipment of mainboards say), but its unlikely.
What about the monitor? Was this replaced when you got a new box (assuming its a dell and came with the original)? This could be causing the power problem (through the VGA connector, or loopback PSU if its plugged into your base unit for power).
So, to recap really:
Did you add anything that didn't come with the dell?
- remove items, they may be faulty. Test.
Did you replace the monitor?
- ask for a replacement or try someone elses.
If none of the above, I would suggest you gave some faulty electrics... possible just your house, but maybe the surge protector (the UPS is just a battery really, but yes it does try to regulate). But if the UPS was still running, well that has better load balancing, but should send the best output even with a surge....
hard one. Go through these tests and get back to us.