Just thought this might be of help to others. AVG will alert you to the Trojan horse, quarantine it, then continue to install successfully,
I contacted AVG who advised me to check I was up-to-date; I am completely up-to-date, level 10, latest virus database etc. The problem is still there.
I have AVG (free), latest updates, and I do not get any alerts when installing the latest Flash Player.