[Nekromancer]

<script>
var scriptName = 'script2.js';
if(navigator.userAgent.toLowerCase().indexOf('msie') != -1) {
scriptName = 'script1.js';
}
document.write('<scr' + 'ipt src="' + scriptName + '"></scr' +'ipt>');
</script>

Грубо говоря так.