MC follows the cursor. How to restrict the MC movement and not the cursor?

programming flash
1

I am trying to make something similar **to this idea **but my MC has a couple of buttons that follow the standard cursor - which is not hidden.
The movieclip movement is working by using:

onClipEvent (enterFrame) {
    this._x += (_root._xmouse-this._x)/25;
    this._y += (_root._ymouse-this._y)/25;
}

At the moment with this code, the button mc follows the mouse over the entire stage. I am trying to figure out how I can limit the movement to a smaller proportion of the stage, by either a hit area or some other method :-/
Obviously I wont want this to effect the normal cursor movement, just the MC that follows it. Anyone give me a clue of how I should go about trying to do this?
Thanks