<div id="my_window" style="display:none; width:200px; height:80px; background-color:#FF0000; position:absolute; left:400px; top:200px; border:solid 1px #333333; padding:2px;">
Hello world!
<br /><br /><br /><center>
<input type="button" value="X" onClick="document.getElementById('my_window').style.display='none';" /></center>
</div>
<a href="#" onclick="document.getElementById('my_window').style.display='block';">открыть</a>