[support] Spam in page header

[george]

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.