No need to use javascript, use a conditional comment:-
<!--[if lte ie 6]>
<link rel="stylesheet" href="ie6.css" title="IE6 styles" media="all" type="text/css">
<![endif]-->
put it AFTER the main stylesheet link in the head section so that it's processed last. lte means less than or equal to so it will only be used for IE6 and previous versions.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.