I reciently built a new computer and it was running great for a while. all of a sudden it stopped starting. the only response i get is my cpu/case fans start for 1/2 sec. my thought was that a capacitaor blowns, but my dad thinks its psu
Well you both could be correct - the power supply has capacitors in it. Your problem does sound like its with the power supply though or there's a slight possibility that something is shorting out elsewhere. I would recommend using a different power supply (borrowing one perhaps) to see if it helps and go from there. If you want to learn more about power supplies go to http://computer.howstuffworks.com/power-supply.htm
