Just be lucky we’re using Invision Power Board as opposed to alternatives, bug filled, hacker magnets like vBulletin or PHPBB.
It wouldn’t have been a virus, like was said earlier, its just spam.
Adam, is the forum running 2.3.6? If not, it really should be. If it is, that is the very latest security update, and last, until Invision 3.0 comes out next month.