Strange fault on new PC

[JuanB]

I am building a new computer for a friend. I'm using his 2 year old OCZ Fatal1ty 550w that worked perfectly on his old pc yesterday.. When I used the power supply today, the computer would not boot (no fans moving, no beep) if there were more than one cable connected at the back. So far I have managed to install windows (external dvd drive) on it with only one cable powering the hard drive on the back of the PSU. I can't connect the optical drive, graphics card etc the PC just won't work with more than one cable connected. Does anyone know what's wrong with it? Is it just the PSU?

 

[twelve25]

It's got Two 25A rails. I suppose one of them could be dead.

 

[JuanB]

Thank you for the quick response. As it is under a warranty I will get my friend to replace it with a new model and see if that works.