Third-party plug-ins, which are installed in the mediacore folder can cause After Effects to stuck at "Initializing Mediacore" during launch. Try renaming or removing the plug-ins from the mediacore folder (except dummy.txt) and see if it brings any change. Location of the folder:
Windows - C:\Program Files\Adobe\Common\Plug-ins\7.0\MediaCore
If it doesn't help, try uninstalling the plug-ins from the Control Panel.