The mysterious story of people only using 10 percent of their intellectual powers is completely false. However, the idea behind it is not, so the reason why people always take it the wrong way is a simple misunderstanding of the theory behind it.
The human brain works like a computer processor: It performs single reflection operations in order to build larger reflections, just like a program on the PC uses simple operations one after another to fulfill its purpose. The number of simple operations a processor can make determines its "computing speed". Recent estimations as to the computing speed of the human brain state that a processor which could match the speed of our brain would have to be a 1680 Ghz processor! (Current top speeds in processors are about 18 Ghz.)
Consequently, our brain should be able to calculate enormous amounts of data in milliseconds, as an average PC today can do this easily, too, and our brain is that much better. And here's where people get it all wrong. Theoretically our brain speed is sufficient for such operations, yes. And theoretically, our memory capacity should range somewhere from 500 to 1000 terabytes, researchers estimate. Why can't we then memorize (for exams) a 500 page book easily, or calculate in a second the first 1000 digits of Pi?
We can't work like computers because we're human. Our brain was never made to calculate stupid mathematical operations or memorize blindly pages of text, so that is not what it's doing. We are a creative and wonderfully artistic species, and as such, we possess brains which help us to get through the day by managing our emotions, developing social skills in order to get along with our entourage and pouring over philosophical questions to explain us why we live. All these operations may seem irrelevant in a class paper, and they are exactly those things which keep us from being super-computers, because all the billions of neurons constantly working on our emotions could also work as a math program, but they're the only thing which truly renders us human.No PC will ever be able to create art or literature, so PC's have advantages and we have ours, but this doesn't mean we're less powerful than PCs!
The 10 percent of our intellectual power we may be able to harness for thinking consciously (while doing maths, reading, etc...) may not sound like much, but the whole machinery behind the scenes is a wonderful product of nature and can never be equalled by artificially made technology.
In the end it seems clear: For those who want to be soulless machines able to calculate very fast: Try to be a PC. For those proud of being humans and USING those computers: Stay human and be proud of the enormous amount of processing power you invest in "irrational" and "irrelevant" activities such as your emotions and art!