wordpress virus

hey guys I've been getting this colbaten ad virus on my site recently, and I've disabelled on my ads on header.php, anyone know what this is or if it's a virus?

Every time when a visitor with Nod 32 visits my site it says that my website is infected with HTML/ScrInject.B.Gen virus I scanned it with sucuri(dot)net and it says: Anomaly behavior detected (possible malware). Details: http://sucuri.net/malware/malware-entry-mwanomalysp8 <script...

Now for the second time I've been stung by a very annoying Wordpress Virus which posts 'automatically related posts' at the end of each posting. Last time I had to pay someone to get rid of it but I'd rather know how to do it myself and also how to keep it out! This is incredibly annoying...