Studio XPS 8000, won't turn on, yellow light on motherboard

Could be either failing psu or bad ram etc. testing w another verified working psu is quickest way to tell whether psu is the culprit.  For ram you need to remove all sticks and put only one back in DIMM 1 (and test all sticks individually until it can POST). During test disconnect hdd, gpu, and all other devices connected to motherboard except monitor to on board video port.

Only PSU I have hanging around is a 300w Lite-On.I hooked it up and same results.Solid yellow light on motherboard.

I used the Dynex psu on the computer I took the 300w psu from.The Dynex works with the computer.What else is left but motherboard/cpu?

Since you said the replacement psu works in another pc, you have ruled out psu issue.  next thing is to test ram. remove all ram and then put only one back each time to test.  if you have another verified working DDR3 stick of the same or similar speed then use that to test by itself (after removing all the ram from 8000).

Tried that,still no change

Still don't know if it is the PSU or not.The current PSU in the XPS 8000 is 520w the one in the other tower is 300w.The dynex 520 might be strong enough to power the tower with the 300w psu due to the specs being lower than the 8000.

No I do not think you need a high power rating psu to just turn the pc on and get basic video.  If your other psu is a working pull from another pc and rams are all tested, try a ddr3 desktop ram pulled from another working pc. If still no change, reset CMOs jumper on motherboard. And you can try remove CMOs battery then drain all residual power from pc.  Wait 5-10 min or overnight. Replace battery or get a new CMOs battery ($2). https://www.dell.com/support/article/en-us/sln284985/how-to-perform-a-bios-or-cmos-reset-and-clear-the-nvram-on-dell-systems?lang=en

Replaced the coin battery.Cleared the CMOS and it would boot into windows.I did a couple of updates,resterted without a problem.Then I turned the computer off,then tried turning it on again and nothing.While the computer did bot-up all fine after the I cleared the CMOS,the yellow light was still on.

seems clearing CMOS helped.

re: Solid yellow light on motherboard.  that is perfectly normal.

I shouldn't have to clear the CMOS just to cold boot the computer everytime I use it.Not much help at all.

No you should not need to clear it each time.  Have you replaced CMOs battery w a new one ($2)?  I think you said you replaced battery.  When CMOs is cleared and battery is good it should keep its settings when pc is powered off and remember the settings on cold boot, because it is supposed to be non volatile memory.  If your pc can not do that, something could be wrong w the CMOs chip.  Question: if you keep computer plugged in wall, are you able to boot pc repetitively?

I decided to connect a power button from a custom tower in the XPS 8000.It powers on but the power button flashes and I get 6 beeps in a row and this repeats.

New battery.Once you shut down the computer,it will not cold boot.

Is this a motherboard failure or a power button failure? Both?

6 beeps in Alienware pc would indicate video card problem. 

XPS 8000 setup guide:  Beep Code Six: Video card or chip failure

p.31/72 https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_studio_xps_desktop/studio-xps-8000_setup%20guide_en-us.pdf

your cpu is lynnfield which does not have iGPU so your motherboard has to have a discrete video card.