Use at your own risk. Backup first. This works for me, but there is no guarantee it will work for you.
This report pulls information from orders_products instead of products to display what products have been purchased, and how many have been purchased. This report adds the option of specifying a date range, so that you can see how many of each product has been sold over a specific period of time.
Just replace admin/stats_products_purchased.php with the new file.
Support Thread: http://forums.oscommerce.com/index.php?showtopic=118737