Hallo Martin,
<HTML>
<BODY>
<SCRIPT>
var fenster;
fenster=window.open("about:blank","lala");
function mach() {
fenster.location.href="http://hier/kommt/neuer/url";
}
</SCRIPT>
<FORM>
<INPUT TYPE=BUTTON NAME="a" VALUE="Klick mich" onClick="mach()"></BUTTON>
</FORM>
</BODY>
</HTML>
Gruß
Hans