You've been given an answer, and it's perfectly valid, but here's something from a different perspective (less strict).
A circle is really just a particular case of an ellipse. Take an ellipse, and change it, by moving its focal points closer together. When those two points coincide, what you get is a circle. It's still an ellipse, technically - one that happens to have both focal points in the same place, is all.
So yes, you can actually have planetary orbits, or any orbits, circular. There's nothing forbidding that. It's just pretty unlikely that this will occur via a natural process.
As indicated elsewhere, in the real world, all orbits and trajectories are a bit imperfect due to perturbations - whether they be elliptical, circular, parabolic or hyperbolic, they are always a bit perturbed by external factors. In many cases, perturbations are so tiny that you can ignore them.
When a planet is orbiting the Sun, and the orbit is elliptical, the Sun will be in one of those two focal points; the other point has no particular signification. If you could circularize that orbit, then the Sun would be in the center of the circle, of course.
Kepler's laws remain valid for a circular orbit:
- The orbit of every planet is an ellipse with the Sun at one of the two foci.
Still true. A circle is an ellipse where the foci coincide.
- A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
Still true. On a circular orbit, the planet moves at constant speed, so the swept area remains constant per time.
- The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.
Still true. The semi-major axis becomes the radius of the circle.
You must understand that Kepler's laws now have more of a historic interest. They are not exactly at the bleeding edge of science anymore. During Kepler's time, it seemed reasonable to state that all orbits must be elliptical (in the strict sense of the term), but now we know that trajectories (including orbits, or closed trajectories) can be circular, elliptical, parabolic or hyperbolic, depending on a few factors.
We also know that perturbations actually deflect all these trajectories a little bit from ideal shapes (but it's usually a very tiny effect).
We also know that relativity makes all "elliptical" orbits more complex - they remain close to elliptic, but the whole ellipse keeps turning around the central star very slowly.
All this stuff was not known during Kepler's time, so take his laws for what they are - a snapshot of the development of our understanding in time.