Computer not powering on. Powers on then cuts off after a second.

Computer issue:
-Computer working perfectly fine last night.
-This morning to move computer off sleep mode, it starts powering on,
then off again after a second.
-This goes on for a few minutes, can not turn off computer normally so
I cut power.
-Try to turn it on again after a few more minutes but previous issue
continues.
-Cut power again, this time unplug everything. Move to
another surface let it sit.
-After an hour goes by, plug in power cord, noticed green light in
back is on so I hit power button, fan starts going like it is starting
up, I plug in monitor, see nothing on screen-Noticed that green light
in the back has turned off and light on power button is orange.-Turn
off computer by holding down power button and let it sit for about 30
minutes.
-Attempted to turn on computer again, goes back to powering on and
shutting off again.

Computer type: Dell Inspiron (not sure on exact model will have to look it up.)

Notes:
-Usually leave computer on due to always working on projects.
-Had computer since summer 2010
-Never open up side panel to go inside it before.

I've looked up the problem already and I've read that it is most
likely power supply unit, but could also be mother board, and I have
even read one account of it being a static shutdown.

Any ideas?