CSS: Position loading indicator in the center of the screen

How can I position my loading indicator in the center of the screen. Currently I'm using a little placeholder and it seems to work fine. However, when I scroll down, the loading indicator stays right in that predefined position. How can I make it follow the scrolling so that it always sits on top??

#busy
{
    position: absolute;
    left: 50%;
    top: 35%;
    display: none;
    background: transparent url("../images/loading-big.gif");
    z-index: 1000;
    height: 31px;
    width: 31px;
}

#busy-holder
{
    background: transparent;
    width: 100%;
    height: 100%;        
}