[support] Spam in page header

Hello,

I've recently noticed when viewing the source of my website, I have 'spam' in the top of the page:

<span style="height: 1pt;width: 1pt;position: absolute;overflow: auto;"> <a href="http://arthistoricum.net/blog/page.php?page=zocor-vs-lipitor.html">zocor vs lipitor</a> <a href="http://arthistoricum.net/blog/page.php?page=what-is-lipitor.html">what is lipitor</a> <a href="http://arthistoricum.net/blog/page.php?page=what-is-zyrtec.html">what is zyrtec</a>

It goes on from there, but I'm concerned as to how this may have happened, and what I can do to fix and prevent it from happening again.

Any ideas? I'm using Drupal 6.13.