It's thought that when Venus was formed it had a rapid Anti(Counter) clockwise orbit like the other planets, but in fact over billions of years, the Earth itself may well have exerted tidal forces on Venus's dense atmosphere, causing it to slow down and eventually, start orbiting the sun in a retrograde direction.
Sorry, but who ever answered above me is just.. Wrong. Venus doesn't move in a clockwise directions, all of the planet's orbits are the same, they move in the same direction around the sun. As for why it ROTATES clockwise, it actually doesn't. Venus' axial tilt is 177.44 degrees (or something close to that, 177 is right). Which means that, while standing from the north pole of the sun, all planets appear to be rotating clockwise except for Venus, which is actually still rotating clockwise, but the planet itself is upside down.