Worm Pierced my Anti-Viral

I’ve been unable to download any mail in Outlook Express today. Instead my inbox is full of endlessly replicating emails from various senders. The box is so overloaded that the mail server has cut me off. I’ve run Symantec scans, McAfee scans & independent web scans. My mail & everything else reads out “CLEAN”. Obviously it is not. Which worm/virus might this be?