OK thanks to the combined assistance received the compromised address book problem seems to have gone. Nothing since the measures various carried out.
I had a web page redirection problem this week but have sorted that out.
However a new game appears, once again I have no idea.
Whenever I am sent a mail with a movie attachment, .WMV or similar, not a YouTube link Thunderbird downloads multiple copies, 47 of one movie last night. BTYahoo's inbox only shows one copy and that as read, but T'bird just keeps on downloading. All other attachments are treated normally.
XP media edition, Thunderbird 12.0.1, and BTYahoo as Mail provider.