There's no need to deactivate the MS Security Essentials. If KIAD is working, it means everything is working correctly, but your FSDT sceneries might be outdated, because you chose to ignore a major update in the past and/or you turned off the scenery update notifications.
Download and install the most current version of the FSDT sceneries you have, and the the Stand-Alone Addon Manager at the very end of the process, or just install JFK V2.1 (if JFK is one of the sceneries you need to install) as the last one.
