My PC has suddenly decided to turn itself on by itself, and no longer responds if i push the power button.
The system itself is about 2-3 years old and has been working perfectly during that time. The only thing i have changed is a fresh install of windows 7 a week ago. I have checked the bios and wake on lan is not enabled, nor any other of those types of settings. I have had to just unplug it from the wall to keep it off during the night time and the next morning i plug it back in and wait a minute and it eventually will turn itself on after about 1-2 minutes. I have also not touched anything inside the system since i built it so i know i didnt unplug anything by accident. I'm out of ideas.
This sounds like your On / OFF pushbutton is malfunctioning. It is simply a momentary-contact switch. Pushing and releasing it for a brief connection is the signal that the mobo uses to turn on. But if it is ALWAYS connecting as if you were holding it in, the machine might behave this way.
The best solution is to replace that pushbutton. But an alternative (and also a test for this possible problem) is to open your case and examine closely the wires from the front panel to the pinout connector labeled "Panel" usually at the bottom right on your mobo. Identify the two from the On / Off switch, and disconnect them. Identify the two from the Reset pushbutton and move then to the pins for the On / Off switch. This makes the Reset switch become your On / Off switch temporarily. If it does the job just like a good On / Off pushbutton, you have confirmed the problem and given your self a temporary solution. You just don't have a Reset pushbutton any more.
I checked out the power button and reset button connections as Paperdoc suggested. Reset button worked fine to turn on the computer, but then just for fun i figured i would check the power button again, reconnected everything and it is acting normal... for now. The connectors from the buttons to my motherboard were on solid and not loose when i initially checked so I really have no idea why it was acting this way. Thanks for the help. hopefully it was just a fluke and reconnecting the wires was all i needed to do.