The transfer of my old messages, trays and folders from the Win XP to WIn 8 machines has been successfully sorted out with the off-the-list advice from Jerry Wise, whose help is gratefully acknowledged in clarifying my misunderstandings.
In summary, the PMail application is run as single user mode - just my wife and myself - with all the messages for over ten years sorted into into subject trays and folders, but we use two email addresses/identities. There is no message filtering used as described in the PM Manual Ch. 6, p.72. By using "File\Selective Mail Download" any suspicious titles and senders can be reasonably easily identified (helped by double clicking on any message in the list to look at the header data) - fortunately there has been virtually no spam for two or three years as it seems the web ISPs have successsfully filtered them out. Nor do we use any automatic sorting to put senders' messages into specific folders or trays, nearly all of which are entitled by topic or subject.
The Win 8 message folder was left as the install default C:\PMAIL\MAIL path. The archive transferred by USB memory stick from the Win XP was installed initially in D:\, but later into F:\ PEGASUSMAIL MESSSAGES. Make sure all files/folders properties are not ticked as Read Only.
Make sure the box is ticked for "Save the Pegasus Mail desktop state between sessions " found in Tools\Options\Basic settings", see PM Manual CH11, p.108 [All my boxes are ticked].
I transferred all the address book file pairs xxx.PMR and xxx.PM! from F:\ PEGASUSMAIL MESSSAGES into C:\PMAIL\MAIl.
To make the archive mail visible to PM click on the yellow folder icon "Work with your Mail Folders" which opens the folder heirarchy listed under "My Mailbox". [NB: this action slightly modfies the menu bar Folder to Folders, with different menus]. Click on the new Folders and click on "Add Mailbox to List" function leaving Directory Path marked in Properties for Mailbox I then added in the archive path and name
F:\Pegasusmail Messages
WinXP 26Feb2013
This now enabled me to see in the folders window two mailboxes
My Mailbox
WinXP 26Feb2013
The only minor "problem" now is that all the archive messages are accessible, but dispersed in grey trays (some 200+) with no folder/tray hierarchy as on the WInXP machine. I had to regenerate the original yellow subject folders in My Mailbox and cut/paste, or drag, the archive message trays back into the their original hierarchy in the Win 8 My Mailbox.
I did this in steps, checking that nothing was corrupted by shutting down, rebooting and making sure messages were accessible. This was successful, but left the added mailbox and one empty tray. Neither could be deleted by clciking the yellow Delete icon. Looking with Notepad at HIERARCH.PM it showed that the archive mailbox F:\Pegasusmail Messages was present. Jerry Wise kindly advised that in Folder Manager window, right clicking on the mounted mailbox would disable the mailbox, then one shuts down PM and checks with HIERARCH.PM that this mailbox had disappeared. Re-open PM and check all is working.
Initially I was puzzled when opening archive messages that the Message Header was a dark purple and the information almost unreadable, on the XP machine it was pale yellow; Jerry Wise reminded me that I should check the setting in Tools\Options\Incoming mail\Message reader, where I could reset the colour. Now the old messages are on the new WIn 8 machine, any attachment paths listed at the end of messages will be incorrect.
I have rebooted the Win 8 machine a number of time now and all seems stable with all the archive messages in the original hierarchy and address books transferred from Win XP.
Rodney Fry