Script run on combo

I have a combo with 4 items on a form "form1".

When the user choses the 4th item "item4" I want to trigger a messagebox and when the message box is closed I want to redirect the user to a different page.

I wish to do this without the user submitting the form.

Can anyone point me in the right direction. I no naff all about client side work (another thing to add to my To Do list).

Thanks in Advance

I did something similar:

<select class="quickl_homepage" onchange="goTo(this)">
<option value="">Bank</option>
<option value="">-------------------------------</option>
<option value="sc.asp">a</option>
<option value="lca.asp">b</option>
<option value="Eur.asp">c</option>
<option value="Eur2.asp">d</option>
</select>


and the javascript function:

<script>
function goTo(srcEl)
{

var temp
temp = srcEl.options[srcEl.selectedIndex].value;
srcEl.selectedIndex = 0;

if (temp!='')
{
window.location.href = temp;
}
}
</script>