I just finished a build for my friend with a combination of used/new parts. The used parts were working perfectly in another computer just a few days ago so I'm sure they aren't the problem.
When I power on the system, the all of the fans started turning/lights turn on etc, but after 3-4 seconds the computer turns off for a few seconds and the cycle repeats automatically until I manually flip the switch on the back of the PSU. There is also no POST of the system whatsoever with the screen completely blank.
What I tried:
-unplugged the HDD, DVD burner, video card, and ram
-swapped the RAM into different dimms
same thing happened after I tried these. One interesting thing though is after completely removing the RAM the computer took twice as long to restart after being powered on (7-8 sec vs. 3-4).
From these tests I'm pretty sure it's the motherboard (brand new). The powersupply I have had for 2 years with no problems and was powering more demanding components than what it is doing now. I last used it 2 days ago in my other computer and was working perfectly. I doubt it's the CPU because not having a functioning CPU wouldn't cause your computer to constantly reboot ( I have had it happen before).
Here is a list of my specs:
-Intel Core i3 540
-Gigabyte H55M-S2V LGA 1156 motherboard (micro atx)
-one stick of OCZ DDR3-1333 ram (2GB)
-Samsung DVD burner
-Rosewill 600w powersupply
-XFX 8800GT video card
-WD 320GB SATA hardrive
-Enermax Uber chakra case
Ok... I've taken the motherboard out of the case and am running it on the box now. Without any of the drives in I turned it on and after restarting about 5 times with nothing on the screen it gave a message saying "Bios checksum error! loading 'blah blah blah' " I didn't get all of it because it happened so fast but then it restarted a few more times then it said "DISK BOOT FAILURE". I didn't have my HDD connected so I turned it off, plugged the HDD in, and turned it back on.
As I am typing this it has now restarted about 8-9 times showing nothing on the screen but now it has loaded the OS and it appears to be working. I'm afraid to turn off my computer though because it might not come back lol.
Ok new problem. I don't think taking the board out of the case is what fixed it, but rather waiting longer than I had for the system to boot up. I have to wait about 10 or 15 restarts for the first POST, but after that the system starts up normally. I put everything back into the case and it does the same thing.
Did you try to clear the CMOS with the jumper or by just removing the battery? You never know may a Bios setting causes troubles. Also check if there is a newer version and update it. Something irrelevant now... Do you have any USB hub connected? If yes then remove it and check again.
I had a similar problem not too long ago and it turned out to be a faulty power switch on the case causing computer to switch on/off randomly. I'd suggest you have a look at both reset and power switches on the case.