I don't think you can do it in a query, but I think you can in a form or a report. If I remember correctly you can do summaries of data and have Access calculate totals.
With a query, I think about all you can do is filter out the records without the box checked. You can then see how many records are displayed, which would give you the count. If you're automating this with VBA, I think you could do the query and do some kind of record count function. Unfortunately I don't have time today to play with it.