Different behaviour for hardcoded scripts and saved in a file

[gfergadis]

Hello,
I have a couple problems which I think they need attention from someone a bit more experienced in web developpement than I am.
I want to change the text of an ASP:Label (Server Side), every time the user hovers the mouse on a client-side object.
I manage to make that work with the script I attach below, however the problems raise when I try to make my code a bit more efficient.
Ideally, I'd like to parse the "S" variable to the server like that:
[b]document.getEle mentById('<%=Ma ster.ActiveLbl. ClientID%>').in nerText="'<%'" + s + "'%>'";[/B]

But as we probably all know this syntax is not possible to provide the realtime result I want. Then, if the ideal solution is not possible, I try to include my script in a JScript File so I will not have to hard code all of these Switch,Case in every page of my website. However, even though when they are hardcode, the following code works fine. When I load it with the JScript file, I get a 'document.getEl ementById(...)' is null or not an object runtime script error.
Any ideas???
Thank you in advance, the script's code follows...

<script type="text/javascript" >
<!--
function MouseOver(S)
{
var sStringToShow;
switch (S)

{
case "BuildingVa l": sStringToShow=' <%=MyAppData.Mo useOverBuilding Val%>';
break;
case "ContentVal ": sStringToShow=' <%=MyAppData.Mo useOverContentV al%>';
break;
case "PostCode":sStr ingToShow='<%=M yAppData.MouseO verPostCode%>';
break;
case "Tenancy": sStringToShow=' <%=MyAppData.Mo useOverTenancy% >';
break;
case "getQQuote" : sStringToShow=' <%=MyAppData.Mo useOvergetQQuot e%>';
break;
default: sStringToShow=S ;
}
document.getEle mentById('<%=Ma ster.ActiveLbl. ClientID%>').in nerText = sStringToShow;

}

function MouseOut()
{
document.getEle mentById('<%=Ma ster.ActiveLbl. ClientID%>').in nerText='<%=act iveLblContent%> ';
}
// -->
//change the text in an asp:Label from client side
</script>