Source: http://www.geek.com/science/geek-answers...

By the time real bodies like stars and planets started forming, the physics were unstoppable. New matter entered the system at far too slow a pace to change the overall direction of travel; not only does every planet go around the sun in the same direction, but upwards of 99% of asteroids and other small features do too. The remaining less-than-1% can be surmised to be very new additions to our neighborhood.

The big, big exception to this is Venus, which orbits the Sun normally, but spins backwards relative to its own axis. Possible explanations range from a huge collision to a solar gravitational effect that slowed, stopped, and eventually reversed its rotation. Probably the most widely supported theory says that Venus actually is spinning the right way, and in fact simply flipped its poles and thus seems to be rotating the wrong way! We likely won’t know for sure until we can see through its atmosphere much more completely than we can today. Neptune also rotates on a tilted axis, which is also thought to be the result of a major ancient collision.

These principles of rotation are also why our solar system moves along a mostly flat plane; as one direction of spin came to dominate in the early solar system, the spinning mass naturally flattened out along that plane. Note that this plane of rotation in our solar system does not match up with the overall plane of galactic rotation; the direction of rotation within individual star systems is largely unaffected by the galaxy, but the systems themselves do all orbit the galactic core in one direction. It’s the same principle at work, but with whole systems this time.