Strange Header Appearing in Outlook 98

HI there,
When one of my users recives an email and displays it on his screen, it appears as it should do.
But when the email message is printed, a rather sinister header appears at the top of the page saying “you’re not the only one that can mess”.
My first reaction was that it is a virus. But I’ve scanned his machine and the mail server with the most up to date virus patch I can find and it picks up nothing.
The strange header is also displayed in the same font that the rest of the mail is displayed.
Any idea’s what could be causing this and how I may prevent it?