Thanks, Backdated, for your kind response.
Theroica says: Before installing the Codec 3.0 Package (5,850KB) uninstall previously installed codecs (Divx, Xvid, etc.).
Being a little unsure, I would welcome any advice on how to do this uninstall, and presumably I should make a Restore Point before actually doing anything?
I don't know if this is relevant, but the DirectX diagnostic tool shows that my PC has DirectX9.0c (4.09.0000.0904) and all the self tests show no faults, especially on the sound tests in mono and stereo in both 8-bit 22KHz and 16-bit 22KHz.
Also, perhaps I should have said earlier, in my first posting, that playing DVD movies in WMP used to work ok until I had a problem with Power Director video editing and a technician at Cyberlink told me that after uninstalling it (prior to a reinstall) to go into the Registry and delete the Cyberlink Folder from Software in both LOCAL_MACHINE and LOCAL_USER. I soon found out that doing this also seemed to have deleted my Cyberlink PowerDVD, which disappeared. From this point on my problem started.