One of the 12v rails could be bad(if it has multiples), or the single rail could be going bad...If you have another PSU laying around, try using it..or your best bet is to buy a new one bro.
Edit: If its a stock prebuilt(name brand) pc, and you don't wanna spend a lot of money on the PSU. go with something like this http://www.geeks.com/details.asp?invtid=PS480D-BLK&cat=PWR
They have really low prices and a 1 year warranty.