A traditional computer and a quantum computer perform calculations in extremely similar manners.  So much so, that a traditional computer is theoretically able to simulate a quantum computer perfectly.  This, of course, begs the question: why go through all this effort to construct quantum computers?  The answer, is that though it is possible for a traditional computer to simulate a quantum computer, the sheer amount of processing power by today's standards makes it effectively impossible for this to ever take place.

The best know traditional factoring algorithm runs in  steps. 

In 1994 a 129 digit number was successfully factored using this algorithm on approximately 1600 workstations scattered around the world, taking eight months. Based on this data, it would take roughly 800,000 years to factor a 250 digit number with the same computing power, and (significantly longer than the age of the universe) years to factor a 1000 digit number!

Recently, an algorithm was developed for factoring numbers on a quantum computer which runs in steps

 Based on this, factoring a 1000 digit number would require only a few million steps using a quantum computer.

Obstacles and Research