I have a Dell desktop that is about 2 years old I got l from a friend. It powers off about 3 seconds after the power button is pressed.
Where do I start trouble shooting this? Thermal issues, ram, power supply? Anyone have an idea?
Did it ever work for you?
With only a 3 second run we can eliminate the thermal issues. It could be Ram and it could also be PSU but my guess is PSU. Check inside the case for disconnected wires, shorting, seated ram sticks, etc. Then test your PSU like this: http://www.youtube.com/watch?v=5FWXgQSokF4
The power button could be shorting. When you have it open, note where it is plugged in and disconnect the power switch. Use a screwdriver to momentarily short the pins where it was plugged in. If the PC turns on and stays on, you've got a defective power switch.
Fans blip on then turn off -- think is power problem somewhere.
Standard debugging is to pull all USB devices and network cables off PC. Try again. When it fails, open case and pull all PCI cards including video cards. If it works it will beep a code for 'no video' or fallback to integrated video (google beep codes). If it beeps or works to integrated video then try re-inserting the cards you took out. Sometimes reseating the cards fixes the problem, other times a card is bad and needs to be replaces.
If the pc is still dead with all the cards pulled take out the memory. try again you are hoping to get the beep code for 'no memory'. If you get the beep for no memory then insert the dimms one at a time until you isloate the bad memory.
If the pc is still dead (no post, no pbeeps) when the memory and pci cards are out, then you have problems. its pwoer supply, MB or CPU. usually try swapping them in that order. It's never the CPU, it's more likely the power supply (PSU) than the MB.