onenterFrame = function(){
xx=Math.random()*widthofyourstage
yy=Math.random()*heightofyourstage
line_mc._ x=xx;
line_mc._y=yy;
}
what this does it randomizes xx everyframe to the width of your stage
it also ranomizes yy to the height of the stage
then it moves the line mc to the xx and the yy
causing the line mc to appear at different places on screen..
try modifying it to make it change not in every frame
and make more things move like a scratch for example
p.s. I added two errors for no copy & paste :D if you cant figure out the errors
look up some general AS tuts
good luck =)