If you are logged onto your forum as the Admin, then the link "Go to adminstration panel" will appear at the bottom of every page as you view the forum.
Click on that link
On the left hand side, click on "management" under the Forum Management menu
Then click on "Edit" against the first forum on your list.
It should then show your general forum settings with Forum Name, Description, Category, Pruning, and Forum Status. The virus code should be hiding in your forum description as a serious of numbers. Delete them all and hit the "update" button.