This was happening to me when my PSU was not supplying enough juice to the system, buying a new one did the trick. It might not be that, I suggest trying one from another PC\a friend before buying a new one.
But I don't have any problem booting.. I can get to the desktop, but then it crashes after a while (depends on how long it's been turned off, if it's been off for a long time, it won't crash for about 15 minutes, then it does. If I turn it on right after it's crashed, it crashes again after about 2-3 minutes.)