The Collaborative International Dictionary of English v.0.48:

Ad- \Ad-\ [A Latin preposition, signifying to. See At.]
   As a prefix ad- assumes the forms ac-, af-, ag-, al-, an-,
   ap-, ar-, as-, at-, assimilating the d with the first letter
   of the word to which ad- is prefixed. It remains unchanged
   before vowels, and before d, h, j, m, v. Examples: adduce,
   adhere, adjacent, admit, advent, accord, affect, aggregate,
   allude, annex, appear, etc. It becomes ac- before qu, as in
   acquiesce.
   [1913 Webster]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

AD
       Active Directory (MS, Windows, AD, DS)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

AD
       Analog-to-Digital (D/A), "A/D"


V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

AD
       Authorized Distributor (DEC)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

AD
       Autonomous DOMAIN