The button on power supply of the Dell Optiplex 760 workstation for

Do they have the same onboard video card? It looks like the Optiplex 760 can come with 7 different possible video cards. If they are different and using the wrong driver it will cause the 0xDA BSOD, which relates to virtual memory being too heavily used (unlikely) or a bad system driver (more likely).

By the looks of things, while its a 'server', it seems to be built around a standard ATX case,and a quick look at dell's website indicates its probably a standard ATX - this link also seems to indicate the PSU is shared with desktop class systems. Power supplies die, and 4 years sounds like a plausible time for it to start dying of old age.

PSUs DO have internal fuses, but unless you're an electrician, there's no real point in opening up the PSU. Its a standard, relatively inexpensive component - it'll be faster, cheaper and easier to simply replace it with a new component.