I don't think there's any way you'll show check boxes as such in a
combo box. The nearest you could get would be to use the usual
method for simulating check boxes of a size/format different from
the default in a form by setting the FontName property of the
combo box to Wingdings and in its RowSource use a computed column
to return the relevant character for a tick, e.g.
SELECT IIf([MyBooleanField],Chr(252),"")
FROM MyTable;
That will give you a tick for each True and a blank for each
False, but you won't get a box around them as you can to simulate
a single check box in a form or report by using a text box based
on the computed column.