The PC shuts down right after it's been turned on, no beeps, no error messages, not enough time even for the monitor to turn on. This computer does have a lot of stuff connected to it, but it also has a 400 Wt power supply which I bought yesteray thinking it's a PS probem. (Did the same thing with the old PS as well). Besides it does the same thing even with only RAM and video card connected. The strangest thing about it that the power light of the PC stays on even though PC shows no vital signs. I am thinking that it's the MB that does this (ASUS). Could anyone give any ideas?

Last time I saw a power light without the power being turned on, a friend had connected his power supply wrong.

I've thought of that too, but it's rather difficult to connect a power supply wrong, but I've checked it once again. But thank you anyway.

Could be one of the cables from the psu is bad. That might account for all symtoms. It might be possible something else is shorting it out, but I can't see how you would then get the power light. Maybe someone else will have other ideas?