Shh! Make your PC quiet as a mouse
There are also steps you can take to deal with noise from an existing PC. If you have a tower unit sitting on your desktop, you can try moving the machine to the floor next to the desk or under it.
Moving the machine a foot or two away, or placing a desktop between you and the noise, can help. The main downside to this is convenience. If you frequently use CDs or your machine has a digital memory card reader, bending down to change media can be a pain.
If your PC sits on the desktop, consider putting a rubber mat underneath. Vibration can echo through a wooden desk and make a computer seem noisier than it is.
But the real cure is likely to involve minor surgery — specifically, a fan-ectomy and transplant.
A new PC that makes a lot of noise probably does so because the manufacturer decided to save money there, but even a decent fan can degrade after a year or so and start whining.
Fortunately, low-noise replacement power supplies are available and not outrageously expensive — from $75 to $120 in computer stores or on the Web. Many use two or more small fans to maximize air flow and minimize the racket. If you're adventurous and reasonably handy with a screwdriver, replacing a noisy power supply isn't brain surgery.
Smaller fans that cool the case and CPU are generally quiet, but sometimes they, too, can make a racket when there's a defect in the shaft that turns the blade or the lubricated sleeve that holds it.
These are easier to replace and relatively cheap ($20 or less), but they come in various sizes, so before you replace one with a low-noise model, make sure it will fit.