var paraOneLeft = 0;
var paraTwoLeft = 400;
var switchDirection = false;

function window_onload()
{
   window.setInterval("moveParas()",10);
}

function moveParas()
{
   if (switchDirection == false)
   {
      paraOneLeft++;
      paraTwoLeft--;

      if (paraOneLeft == 400)
      {
         switchDirection = true;
      }
   }
   else
   {
      paraOneLeft--;
      paraTwoLeft++;

      if (paraOneLeft == 0)
      {
         switchDirection = false;
      }
   }

   para1.style.left = paraOneLeft + "px";
}