js_get_scrollTop()

/**
* Gets the amount the page has been scrolled down.
*
* Credit: http://www.quirksmode.org/viewport/compatibility.html
*/
function js_get_scrollTop(){

if (self.pageYOffset) // all except Explorer
{
y = self.pageYOffset;
}
else if (document.documentElement && document.documentElement.scrollTop)
// Explorer 6 Strict
{
y = document.documentElement.scrollTop;
}
else if (document.body) // all other Explorers
{
y = document.body.scrollTop;
}

return y;

}