It could be anything. I am guessing that you have a poor power supply. It is not uncommon for pre-built computers (which I assume you're using) to come with shoddy power supplies. I would suggest looking at your hardware and finding out what the voltage and amperage needed for your hardware is and then finding a power supply online made by a reputable company such as Corsair, Seasonic, OCZ, PC power & Cooling, etc. and buying one through them to replace your existing one. If you don't want to do that yourself, take it to a PC store that can help you with that.