This behaviour is caused by the famous Internet Explorer z-index bug and can be observed only on very few web pages. The Internet Explorer web browser does not comply with the WWW standards and thus sometimes displays the elements incorrectly.
In most cases workaround is easy - simply put the <script></script> part at the bottom of your HTML code just before the </body>
Internet Explorer 8 fixes the problem, however it may still appear in the IE7 rendering mode of the Internet Explorer 8.