From your description, it looks like an antivirus bug: it blocked the installer midway, resulting in a corrupted installation. Maybe it blocked it in the worse possible moment: while installing the MS VC++ runtimes, that's why you got a Side-by-side error.
Try the solutions indicated here at point #2, to restore the MS VC++ 2005 runtimes, that are needed by FSX to run:
http://www.fsdreamteam.com/forum/index.php/topic,7545.msg62899.html#msg62899Then download and install the latest Stand-Alone Addon Manager, which will install the MS VC++ 2013 runtimes too, which are needed by our software modules (and KDEN too)
Then install KDEN
IMPORTANT:
Do this with every antivirus or security software DISABLED. And Restart Windows before starting the procedure.