Why is the rotation of the Earth always counter-clockwise?

After the Earth formed and all the component asteroids and other proto-planetary debris coalesced the residual spin from the collisions left us spinning counter-clockwise.  Were there to be another collision, a series of collisions, or close interaction with some super-massive object (i.e. black hole, brown dwarf, etc.) with enough momentum or gravity to retard, stop or reverse our rotation, it could definitely do so.  Venus is an example of a planet in a retrograde rotation (probably because some massive impact reversed its spin), so it is definitely possible...hell, Uranus spins almost 90 degrees out of alignment with the orbital plane.  Most planets in our solar system spin in relatively the same direction, and that is likely to be the case in most systems in the universe, but it is by no means set in stone.