In my FSX menu I am seeing the entries for Coualt and Addon Manager and in the Addon Manager it shows that the products are active but not in the scenery library.
That's the problem so, you can disregard everything about antivirus, etc.
The most likely issue seems, because the way you updated from FSX to FSX Steam, your registry is not corrupted. Most likely, you installed FSX:SE, then you removed FSX and this WILL cause problems.
The correct way to switch to FSX:SE, is to uninstall FSX BEFORE installing FSX:SE, otherwise your FSX:SE will be installed in "dual mode", assuming it's running together with FSX, but if you remove FSX afterwards, all the registry entries will point to the wrong folders and will confuse products that properly recognize when the two sims work together, like our own.
I think uninstalling FSX:SE and reinstalling it should fix your registry and have the sim correctly configured to run in "single mode", which is how is supposed to work when there's no FSX installed.