<select>
    <option value="Webclient" name="">Web Client</a></option>
    <option value="DownloadClient" name="">Download Client</option>
</select>

<select name="forma" ONCHANGE="location = this.options[this.selectedIndex].value;">
 <option value="Home.php">Home</option>
 <option value="Contact.php">Contact</option>
 <option value="Sitemap.php">Sitemap</option>
</select>

<select id="options" onchange="optionCheck()">
<option></option>
<option value="Webclient" name="">Web Client</a></option>
<option value="DownloadClient" name="">Download Client</option>
</select>
<script type="text/javascript">
function optionCheck(){
    var option = document.getElementById("options").value;
    if(option == "Webclient"){

        window.location = "http://yahoo.com";
    }
    if(option == "DownloadClient"){
        window.location = "http://google.com";
    }
 }
</script>