I’ve been hacked

Ok, several of my recent posts have been hacked – the following gets inserted into my posts and is only visible when looking at the HTML in the editor

<span style="position: absolute; top: -8349px; left: -3208px; display: none; text-decoration: underline;">new today offer -
<a href="http://www.scoilacla.com/images/gallery/.all/prod_id_436.html">tramadol drug interactions</a>
..... many more similar arefs......
</span>

The site scolilacla.com seems to be a valid site – but appears to be hacked itself.

What should I be looking for to figure out how this is happening and how to fix it.

Thanks in advance.
Bob.
(SPAMMERS SHOULD BE LOCKED UP FOR LIFE!)