PC not switching on

Im having problem with my pc not turning on, the only thing showing any signs of life is the green led on the mobo, everything else, fans, hdd etc are all lifeless. Not sure if its the psu or the mobo, unless you can think of anything else it could be.
I have done all the tests that I have found on different sites, removing all but main parts, bypassing the power on button, crossing terminals 14 & 15 on the power connector etc, and also checked for any signs of burn outs and funny smells but cannot see anything as to what the problem is. Please help as on limited budget, so dont want to start buying parts only to find out it is something else.

PC specs. 2007 Mesh computer. Intel core2 duo E6700. Asus p5n-e sli mobo. nVidia geforce 8800gts gpu. Running winXP and win7 on two separate hdd
 
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