Well, the equation of a circle with radius R on the origin is [TEX]x^2+y^2=R^2[/TEX]
All you have to do is test if this inequality is true or false : [TEX]x^2+y^2 <= R^2[/TEX]
If it is true the point (x,y) is in the circle, if it is equal it is on the circle, else it lies outside the circle.
It should be rather easy to write that in C#.
Don't know exactly what you mean here but I guess it is something like this [TEX](x-a)^2+(y-b)^2 = R^2[/TEX]
Where a and b are the new coordinates of the center. If a and b are both zero then the circle has its center at the origin.