The Purge Notes function (for customers, vendors, items, or accounts) should only be purging the detailed notes with time/date/user information. It should not be purging the Additional Information at the top of the screen.
The current shipping version and service pack of Adagio Receivables (6.8B - May 7, 2004) does not clear the Additional Comments when notes are purged, even if all notes are selected to be cleared. There have been no reports of this happening with other clients.
If you haven't already, use the Customer Inquiry function to verify that the Additional Comments are still there. If you are using the Notes report to check what has been purged, an issue was fixed here with regards to printing the Additional Comments. In any case, the most current service pack should be installed.