var xPos = 20;//from alixixi.com
var yPos = document.body.clientHeight;
var step = 1;
var delay = 30;                     
var height = 0;                     
var Hoffset = 0;                    
var Woffset = 0;                     
var yon = 0;                    
var xon = 0;                     
var pause = true;                    
var interval;                                  
function changePos() 
{                   
    width = document.body.clientWidth;                    
    height = document.body.clientHeight;                    
    Hoffset = aaimg.offsetHeight;                    
    Woffset = aaimg.offsetWidth;                   
    aaimg.style.left = xPos + document.body.scrollLeft;                    
    aaimg.style.top = yPos + document.body.scrollTop;                    
    if (yon) 
    {                  
        yPos = yPos + step;                    
    }               
    else 
    {             
        yPos = yPos - step;                    
    }            
    if (yPos < 0) 
    {                  
        yon = 1;                    
        yPos = 0;                    
    }                  
    if (yPos >= (height - Hoffset)) 
    {                    
        yon = 0;                    
        yPos = (height - Hoffset);                     
    }                  
    if (xon) 
    {                    
        xPos = xPos + step;                    
    }                 
    else 
    {                 
        xPos = xPos - step;                   
    }                  
    if (xPos < 0) 
    {                    
        xon = 1;                   
        xPos = 0;                   
    }         
    if (xPos >= (width - Woffset)) 
    {                    
        xon = 0;           
        xPos = (width - Woffset);                    
    }                    
}
function start() 
{  
    aaimg.style.top = yPos;                      
    aaimg.style.visibility = "visible";                  
    interval = setInterval('changePos()', delay);                    
}