5. Upgrade to a quiet power supply.
Your computer's power supply may be one of the primary sources of noise. Many rely on two built-in fans to move large amounts of air. Switching to a power supply that is designed to be quiet can dramatically reduce your PC's overall noise level. Designs vary, but most units simply employ larger fans that can turn more slowly while delivering the same amount of cooling air. Some also use thermostatic controls to slow down or speed up the fans depending on the case temperature. For instructions on upgrading your power supply, see "Keep It Powered, Keep It Cool". Be sure to buy a power supply that has sufficient wattage to handle all of your PC's components.
6. Install a new CPU cooler.
Today's processors run very hot--often from 145 to 175 degrees Fahrenheit. At these temperatures, effective and continuous cooling is essential. In fact, an uncooled CPU can grow hot enough to damage itself in a matter of seconds. The CPU heat sink and fan that came with your PC are probably louder than you realize. You can lower the noise by installing a specially designed CPU cooler. Most coolers include a larger, more efficient heat sink that can be paired with a much quieter fan. Buy a cooler that's designed for your processor, and follow the manufacturer's directions for installing it.
7. Install acoustic insulation.
If your PC still isn't quiet enough after you've taken the preceding steps, try some more-extreme measures. Acoustic insulation kits let you add a layer of special sound-absorbing foam on the inside of your case. Installation is relatively easy: You cut the foam to size, peel off a backing, and stick it in place. The SilentDrive enclosure isolates your hard drive in a sound-deadening box. You'll need a free 5.25-inch mounting space for each SilentDrive. Follow the manufacturer's directions to install it.