I had this problem problem before
You have two choices, its the psu, as they said before, or you screw up the BIOS. If you didnt overclock anything, you didnt put any new hardware, etc. There's a good chance its the psu that is getting old or is being overused by draining periphericals (like a new video card)
This is what I read, me my problem was that I had crappy Ram
Conclusion: Try loading BIOS defaults
BTW, to chek if your psu is ok, get a lil program, like speedfan or anything like this (or even maybe in the BIOS) to chek if u voltages are correct. The real value should not be over more than 10% and not below 25% of the nominal value
gl hf