How to build a quiet computer from scratch?

Read this excellent post

http://www.codinghorror.com/blog/archives/000665.html

1. The easiest way to build a quiet PC is to start with components that run cool.

2. Minimize the number of fans in your system.

3. Control the speed of your fans.

4. Consider aftermarket cooling solutions.

5. Dampen your hard drive.

6. Use noise-reduction materials.

7. Passive cooling isn't worth it.

CONCLUSION

The best way to quiet your PC is to begin with the right parts. So use:

1. CPUs and video cards that run cool
2. a quiet, efficient power supply
3. hard drives that run relatively quiet as shipped

You most likely have to PAY to make your machine quiet by paying for better engineering and higher quality components.

Silent PC Review (SPCR) is the place to go for for this.