Pc shuts down after being powerd on

Hello,
I have a packard bell stationary with a really annoying problem after powering on, the machine just goes off anywhere from a second to 30 seconds. ive tried new PSU, HDD, Completely flushed the insides with a compressor reseated all possible hardware, nothing.... I found out that when the machine shutts its-self off the keyboard light is still lit and if i hit any key on the board the machine will Once again come to life and quickly die off. Also just plugging the Ps2 keyboard inn will trigger the machine to power on :S

Im seriously stoked and have ran out of knowledge.. If any1 could offer any additional Advice or if anyone has come accross this kind of problem before.. Would really be of help

p.s When the machine shutts off all fans including the PSU itself turn off,

Lastly it seems the longer time i leave it switched off the longer it will stay on for me, but once it goes it pretty much goes instantly if you was to try and power it back on right away

cheers for your time
Thomas

Look at Power Options in Windows Control Panel