My computer is very loud and the noise is comming from my fans. I think the powersupply fan is very loud. How can i make this fan run more quietly?

you can't on the psu, unless there is a POT. get a new psu with variable POT.
for the pc fan, get a bigger one. bigger spin slower but pushes more air

Hunt around for a replacement for that PSU-fan from fan-makers like Pabst or Zalman. With a bit of careful DIY you should be able to replace that noisy fan. Depending on model, you might need 2 or even 3 fans.
Most fans are standard size 80, 90 or 120 mm.
It's propably cheaper and easier to buy a new, quiet PSU with 120mm fan.
Get at least a 430W PSU from a wellknown brand such as Enermax, Antex. Cost a bit more, but well worth it.

Should be noted if you swap out a PSU fan......
DONT TOUCH ANY OTHER BIT INSIDE THE HOUSING!
Unless you want curly frizzy hair or intend to audition for an Al Jolson 'look-alike' comp... :eek:
Them thar capacitors hold quite a jolt even after you've disconnected all sources of power!

Very true, in fact, I'd call it more then a jolt, they can kill. But if you disconnect the PSU from the wall and let it sit for a few days, the caps should discharge on their own. Even still, do be very careful, I'd recommend wearing rubber gloves.

Most Antec power supplies have built in Low Noise technology that can control PSU fan speed as well as the case fan(s) speed in response to case temperature.

If you're going to buy a new PSU, you could as well buy a passively cooled one. Naturally, they're more expensive, but hey, at least they're silent, not just quiet.

I recently change my processor fan to a new Cooler Master fan. Unfirtunately it still makes a lot of noise when I turn it on (it runs at 100%). I use the program Speed fan to control the speed but I am wondering whether I can permanently change the fan speed so that i doesn't make so much noise...