it is done by using css styles, you can try something like this:
function hover(node_id)
{
document.all[node_id].style.backgroundColor = "#cccccc";
document.all[node_id].style.borderTopColor = "#999999";
document.all[node_id].style.borderLeftColor = "#999999";
document.allnode_id].style.borderRightColor = "#999999";
document.allnode_id].style.borderBottomColor = "#999999";
}
and assign each TD an unique id (node_id):
<TD onmousemove="hover('node1')" id="node1">
...
</TD>
of course you'll have to add an "onmouseout" handler to clean those TD's...
this is just one of many ways to do this, M$ uses css behaviors