This article is from the Viruses and the Mac FAQ, by David Harley D.Harley@icrf.icnet.uk with numerous contributions by others.
From time to time there are reports from Mac users that the message
'Welcome Datacomp' appears in their documents without having been
typed. This is the result of using a Trojanised 3rd-party
Mac-compatible keyboard with this 'joke' hard-coded into the
keyboard ROM. It's not a virus - it cannot infect anything. The
only cure is to replace the keyboard (be polite but firm with the
dealer if you were sold this as a new keyboard!).