Changing The Url Of An Iframe With A Selection Box

I have been wrestling with a problem, I am trying to change the url of
an Iframe from a simple selection box, once the user selects the
desired url they wish to visit from the main page the iframe then loads
the url while not affecting the main page. I am using the onchange
event of the select tag to drive the javascript function hopefully
passing the value of the selection wich in this case is the url to the
independant script wich will in turn will change the src of the iframe
from the default page that is loaded when first viewing the page.One
question I have is would it be better to use the onload event to set
the initial src then use the onchange event form the select to change
the src or staticly set the src then use the onchange event to change
it? below is an example of my code so far. Also currently whenever I
try to pass teh value to the script I get an object expected error and
nothing happens.Bear in mind I know enough C++ to make myself dangerous
and can stay on top javascript to a point but right now I am just lost.
just need a nudge in the right direction at this point please :D

<select name="sites" id="sites" onchange="changesite()">
<option value=" "> </option>
<option value="http://www.nanpa.org">NANPA.org</option>
<option value="http://www.google.com">Google.com</option>
<option
value="http://www.spywarewarrior.com">spywarewarrior.com</option>
</select>

Function changesite(value)
{
var newsite = value;
document.frames['websites'].src = newsite;
}