Everything is "vulnerable" to a virus. I don't know if there
are any viruses that specifically target AD but their will be
eventually (you can search Google or better the Symantec
anti-virus info site.)
A virus could attack the files directly (Del "%systemroot%\sysvol" /s /q)
will work on many machines if AD has not yet locked the files on boot
and the virus is running as Admin or System.
Running any (downloaded) script that knows ADSI or Ldap could
be a "bad thing" for AD.
