OK, some very strange things are happening here, but it looks as though my sentence will be commuted to life misery. The search for .dbx files found a lot of old messages in my spare hard drive of all places, and that is somewhere I haven't visited for at least twelve months. Ours not to reason why, however.
After importing said files, what all folders have in common is that the last message is on Nov 11, which my feeble brain finally worked out to be exactly three months ago. Presumably an option is set somewhere involving three months. I don't know, it's all beyond me completely, especially finding them in the D: drive.
I was going to download the extract file mentioned by ORAC, but alas, one other small complication since the install is that I can't get the internet on the desktop. All the configuration is correct, Device Manager indicates the network card is working properly, but the poota keeps telling me that a network cable is unplugged, and it bloody well ain't! I have double checked all connections, replaced cables, and I still get the same indication. My router is working fine on my lappy, and the four port switch I use indicates the connection to my desktop is fine, green lights everywhere, but no, something is unplugged.
Had a brainwave and reinstalled modem/router software: nope. Yet oddly enough, nowhere in Device Manager do I see the router mentioned.
Confusion reigns.
Hang on. Another brainwave has just struck me. I shall report back with a progress check.
Grrrr. Nope, that had exactly the same effect as all my other brainwaves. Nada.