Computer and fans turn on for 1 second then turn off

I’ve searched through the threads but i am still unsure of what my computers problem is.

the fans and the lights turn on and then turn off right away. I have tried a new motherboard but that didnt work. i tried a new processor but that didnt work. I was wondering if it was because there was not enough thermal paste but the problem still happened when i put a new processor on with new thermal paste.
what else can cause this problem? my pc has been working fine for over a year now and this just happened . havent done any changes to the system.

specs:
AMD Phenom 9500 Quad-core
ATI Radeon HD 4850 1GBDDR3
2GB DDR2 Ram
450w Power Supply
300GB Seagate HDD
Windows XP Pro 32-bit Service Pack 3

Thank you very much in advance.