I am thinking about ot keep advertisement in such a way that when someone scroll down my page that add should go down too. In short it should be visible all the time. SOmeone told me that it is possible using javascript. Anyone know how to do it?

yes it is possible by the java script.but it is a long script so i can not written all the script over here,but it is easy to made so i can give u the logic.just u have to make a funcation that contain the timer loop ,that is running upto the ending of ur page.