Some fans are really loud, and I don't think yours is as loud as some I've used
You can easily change the fan over to another one thats quieter, but you'd have to be sure which fan is the noisy one first, and replace that one (or all the noisy ones).
If you have a cheap power supply, chances are that the fan is in the power supply itself. In that case, you might have to buy yourself a new power supply. Or change the fan in it yourself, which may be dangerous if you don't know what you're doing.
www.pccasegear.com has a list of recommendations for quiet fans. You should be able to find some fans locally which are on that list. Bear in mind you'd have to look for that exact model, as the same makers may make very noisy (but very powerful) fans as well.
Also, if you're using the stock Intel CPU heatsink and fan, you will need to change the whole heatsink and fan to quieten the PC. For that, there's a whole list of aftermarket CPU coolers. For your use, I think the Zalman 7700 should suffice. Its not the best around, nor the quietest, but its very good value for money.
