PSP gives you two options for creating a circle - Circle and Ellipse
If you select the "Draw Circle" option it'll always correctly constrain the X and Y radius to be equal creating a "perfect" circle.
You can also simply hold down the Shift key as you drag out a circle, this will also constrain the X and Y radius.
As for what these values mean: Center X and Y refer to where the center of the circle is on the image. A Center X Y of 800 and 600 will place the center of the circle at pixel location 800x 600y.
As you know the "radius" is the distance from the center of the circle to any point along its edge. So essentially how big the circle is. If X Radius and Y Radius are different you'll get an ellipse (oval) because it's not the same distance to one edge as it is to another.
If the X Radius and Y Radius are exactly the same, so it's the same distance all the way around, you'll get a "perfect" circle.
So if you find that you've made an oval and want to fix it later, just make sure the X and Y Radius values match.