Chek Windows Vista using Javascript

Starx

By using Javascript try
alert(navigator.userAgent));

However I am not sure if, it is cross browser. Better way would be to use server side languages.

By using PHP, you can do this.

<? echo $_SERVER['HTTP_USER_AGENT']; ?>

Both, Javascript and PHP outputs something like this

Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.12) Gecko/20101026 Firefox/3.6.12

You can detect the OS using this output as Windows NT 6.1 is Windows 7