Auto hide (movie clip) task bar

How do I make a auto hide task bar (which is a movie clip)?

auto hide bar similar to windows task bar, when move mouse to bottom of screen, the task bar come up, else task bar auto hide.

I try to set code in movie clip (task bar) but don’t work well

on (rollOver)
{
_root.top._x = 400;
_root.top._y = 220;
}

on (rollOut){
_root.top._x = 400;
_root.top._y = 150;
}