Hi. I am new here, and I hope I have posted this in the correct forum.
My problem with my pc is this..
A few days ago, I left my pc on while asleep <i often do this> I woke up the next day, it was off, and would not turn on. I diagnosed this as a problem with the power supplu, so I bought another 400w for my p4 2.4 ghz with 700 or so ram in it. On installing it, it ran fine. I left it on all night , and awoke the hext day with it still running. I then shut it down before going to work. I come home that night,turn on the pc..it begins to boot up..windows booted sucessfully and such..upon opening my windows messenger app, the pc..just died, as if someone had pulled the plug on it. The fan was making a slight odd sound <in my opinion> the night before,so I thought maybe I had a FAULTY POWER SOURCE. Upon replacing the 400w source again, it still will not turn on. It must be getting a small surge of power, because the fans will get 1-2 rotations in..but still nothing. Every thing looks like it is connected just fine, and nothing looks damaged when i looked in close. There are no strange smells or such to report, only that it wont turn on. Anyone have any ideas what it could be? Many thanks.

Welcome to http://www.pcguide.com/ubb/pcgubb.gif :D

Ok, as to your problem, I'm thinking your CPU fan might have gone bad. When this occurs, your CPU gets REALLY hot! But the P4 has built-in protection, so instead of just frying (like AMD procs), it shuts the system down. That's my guess so check that first.

Bad CPU fan sounds likely to me too, and check for Bad Capacitors (http://www.pcguide.com/vb/showthread.php?s=&threadid=25482). Capacitors seems less likely, but this sounds like the kind of thing that will happen, and it's free and takes only a couple of minutes to check.

thanks for your helpful replies....I ended up tuesday night taking it to compusa for a good look, im still waitin for a response, as there are 15 pcs in front of mine to be fixed;p
Capacitators and cpu fan inever considered. I am still new to the innards of the pc, so to speak. Thank you so much guys, I really appreciaite it:)