determining browser then performing function

A little quest I am on if I may intrude.  I have the following script
(received from this gracious list) that helped me see the browser and call
the appropriate style sheet.  I would like the same ability to call a
function.  In a nut shell, if netscape do this "function popupWin() {" if
not then ignore it.  Can the below script be altered to do that or does
anyone have another handy that can do the job?  Thanks!

<SCRIPT>
if (document.layers) //If Netscape
{
document.write("<LINK REL='stylesheet' TYPE='text/css'
HREF='../scripts/nsbody.css'>");
}
else if (document.all) //IE 4 & 5
{
document.write("<LINK REL='stylesheet' TYPE='text/css'
HREF='../scripts/iebody.css'>");
}
</SCRIPT>

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