Many thanks Dadgad - I have also just added two extra tags to the thread: 'autocad', 'virus' to help others find it. I am also considering adding the contents of my above post to the Tutorials section of my site, so that the information can be found and accessed easily.
At Lee's suggestion here, I read through this thread, and thought I'd share that my office recently experienced an Acad.fas / AcadDoc.fas / BakDwg.fas virus.
My initial thought as well (once the servers, and computers were cleaned up), was to write a LISP function to 'kill' this virus, but how to ensure that my function is loaded first (before those within the project directories), and my limited testing showed that simply registering a .NET assembly does the trick.
Windows Registry Editor Version 5.00[HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\[color=red][/color]\Applications\[color=red][/color]]"DESCRIPTION"=[color=red]"Your description"[/color]"LOADCTRLS"=dword:00000002"LOADER"="[color=red][/color]\\[color=red][/color]\\[color=red].dll""MANAGED"=dword:00000001