usally this happens cuz
a) you messed with the ide connections , reconnected or connected another hd , and you connected the ide connector upside down to the drive/s (hd/optical) or to the motherboard ide slot
b) you reseated the processor from some reason , did it badly and bended some of its pins
c) unstable psu voltages
d) bad mobo
did you do a/b ?
did you add any new hardware recently?
provide full system specs including psu brand/model (look on its sticker)
check the mobo for bad capacitors (its kinda dark in there so better use a flashlight) , see examples