Brain

We actually utilize the full 100% of our brains. Most of idea that we don't is theoretical myth. The brain is not entirely utilized for thought however, which is largely where the whole myth of not using the entire organ is born. Different lobes/sections control different aspects, such as equilibrium and balance, motor control, memory, problem solving, processing the senses, instinct, controlling the various glands within the body, controlling every other organ in the body for that matter, etc.

It is like a computer.

Just because your hard drive and ram do the thinking/data storage doesn't mean that 100% of your computer doesn't work. The rest is keeping the memory and thinking aspects working. Otherwise yes, I can say that only a small percentage of my computers component parts actually "work" when it is performing a process.

As to what specific portions of the brain do specifically what? Much of that is still somewhat of a mystery.

Oh. And if you want a source for further information from the field of neuroscience (the study of the brain and the nervous system), here:

A page called Neuroscience for kids that will explain in detail in terms you can understand.

http://faculty.washington.edu/chudler/tenper.html

Enjoy.