Copy and paste the code below into your main webpage, just before the ending head tag (</head>).
The code will pop under a very small (125x125) window when your visitor comes to your page. Just once per visit. The only thing you have to change is change where it says: YOUR-DOMAIN-NAME.COM to your site's name. (DO NOT include http:// or http://www., just the domain name only.) In addition, if you have more than one domain mirroring the same site, on most systems you can use the variable <!--#echo var="HTTP_HOST" --> instead and it'll automatically insert the domain name the person used to get to your site.