Thanks so much for the prompt reply, Scott.
I've already tried the solutions suggested at http://www.sysnative.com/forums/windows-update/14469-repeated-prompt-windows-update-kb890830.html
, but I had no luck: see my post and the discussion with Aura at http://www.sysnative.com/forums/windows-update/17644-kb890830-win7-update-keeps-popping-up-every-time-even-if-already-installed.html
. In particular, the file dowlnoaded from https://www.microsoft.com/en-in/download/malicious-software-removal-tool-details.aspx
(Windows-KB890830-V5.31.exe) at least in my case, allows only to scan the PC and not to install anything (i. e. I can't get MRT.exe by this way).
Disabling the update is a good idea, I've already tried it, but the only outcome was that disabling december version leads to november version popping up, and disabling november version leads to october version popping up, and so on backwards... I'm sure you caught the idea... (I apologize if my 'ing' forms are not correct, I'm not an english native-speaker).
For now I let the update to show itself and I don't install it, hoping MS will provide a solution in the future.
Thanks again and bye.