You can replace the fan in the power supply, but opening it voids the waranty. If its a cheap power supply, I wouldn't even bother, since you can get a decent one for under $50 that will be quiet without any modifications.
What video card do you have? There are a lot of after-market coolers. If you have an Ati card, you might be able to slow down the fan with just a program, such as Ati Tray Tools, but it might overheat.