DropDown changing URL

OK I tried searching for this but ummmm for some reason im not a good “searcher” for information because i don’t really know what i am looking for :stuck_out_tongue:
Anyways though here is what i need.

When the url is index.php there is a dropdown box:
<select name=‘category’>
<option value=‘1’>Category 1</option>
<option value=‘2’>Category 2</option>

What i need is when the user click on one of the categories it changes the url to index.php?cat_id={chosen category id}

I am using php and html (I don’t use flash :O). I know this is done with javascript but i am ignorant in that area as well. Anyhelp would be much appreciated :smiley: