May be following code will help.
If Rs.State = 1 Then Rs.Close
Rs.Open "select distinct(Your_Field) from Your_Table order by Your_Field", Con, adOpenKeyset, adLockPessimistic
If Rs.RecordCount > 0 Then
Combo1.Clear
Combo1.AddItem ""
With Rs
Do While Not .EOF
Combo1.AddItem Rs(0)
.MoveNext
Loop
.Close
End With
Combo1.Visible = True
Combo1.SetFocus
End If