PHP код:
<script type="text/javascript">
function redirect(folder, page){
top.location.href = folder + '/' + page;
}
</script>
<FORM>
<SELECT NAME="navFolder"<>
<OPTION>Выбор папки</OPTION>
<OPTION VALUE="folder1">1 папка</OPTION>
<OPTION VALUE="folder2">2 папка</OPTION>
<OPTION VALUE="folder3">3 папка</OPTION>
</SELECT><br>
<SELECT NAME="navSelect"<>
<OPTION>Выбор раздела</OPTION>
<OPTION VALUE="1.html">1 страница</OPTION>
<OPTION VALUE="2.html">2 страница</OPTION>
<OPTION VALUE="3.html">3 страница</OPTION>
</SELECT><BR>
<INPUT TYPE=BUTTON NAME="go" VALUE="Go!"
ONCLICK="redirect(this.form.navFolder.options[this.form.navFolder.selectedIndex].value, this.form.navSelect.options[this.form.navSelect.selectedIndex].value)">
</FORM>
Примерно так