Help! This problem started today. When i turn my PC on, it turns off some seconds afterwards. I put it here, because im suspecting its a PSU failure. I bought a XFX 8800GT a month ago, and my system worked ok until now. So, what do you think may be happening with my PC?

 Thanks!

PS: Sometimes it turns off before windows XP load screen, sometimes in XPs boot screen.

Power issue is the most probablt cause. Try running the pc wiht another psu you KNOW works. If it still isnt working then its most likely a overheating issue. Re apply thermal paste to main component, the processor.

Until yesterday, Everest reported normal temps on all (CPU, GPU, mobo, etc.) My system is not OCed(except for the vid card, but its a factory OC). Is it possible than from one day to another the CPU is overheating for no apparent reason?

 hmm loose wires? try switching psu.

PSU's a big possibility.  The other chance is that your system is shorting.  Try taking out nonessential parts and turning on the computer and see if it stays on.  You can even try turning it on without a Video Card.  It might beep and you certainly couldn't get anywhere, but it should not turn off, and if it still does turn off that that means it's not the video card.  It could still be a crapped out PSU problem, but not a lack of power (watts) in that case.

I've seen some strange things cause shorting.  One system would not work and we were ready to RMA stuff before trying booting it after painstakingly removing one part at a time.  It turns out the case's front USB caused the shorting.  Weird.

Thanks, crisis causer. May be you are right. Two days ago my PC started working properly. The only thing i did was change the cable that connected the PSU to 220v. But im not sure if it was a coincidence. Another weird thing that happened to me yesterday was that when i tried to connect the VGA  of the monitor to the 8800GT, my pc turned ON by itself!! Also, my front USB isnt working, because some while ago i dropped some water on it. Now that i think of it, i should have disconnected the frontal USB the moment i noticed it was broken. Im doing it right now.