InTechSpot  

The most important step in creating a quiet PC is without a doubt installing a silent CPU cooler.
By far the loudest component in most PC's is the CPU fan. With higher and higher clock speeds comes the need for larger coolers, and fans that move more and more air.
The main problem with most OEM CPU coolers is this: They are designed with two things in mind: size and performance. This means that they have to be small, so they can be accommodated in the majority of users' PC cases. They also have to perform well at the same time.
The only real solution for this is to use a small fan, usually no more than 60 or 70mm in diameter. Unfortunately, the smaller a fan is, the faster it has to spin to move enough air. The faster it spins the louder it gets.
That's the problem, so what's the solution? There are a few options available, depending on your needs and goals in building a silent PC. We'll cover them separately.
Silent CPU Coolers
The quietest option is to use an entirely fanless CPU Cooler. If you're going for a completely noiseless PC, this is the best route for you.
How does this work? Well, as I said above, OEM designers have to design with size in mind. They don't have to consider those of use who want quiet PC's, and will do what it takes to build a quiet PC. Therefore, Silent PC users have to forego the size consideration, and go with a HUGE cooler, with no fan at all.