In addition to excluding the folder(s) your media files are in, you really should also exclude Media Center 26's folder/files as well.
In my own experience, Windows Defender (along with and other antivirus applications) can cause weird issues in MC, especially after a new, major MC version was just released. It may or may not happen to everyone, but I have noticed things in the not-too-distant past like Defender causing MC to lock up, and higher resource usage (I suspect Defender's engine is submitting MC's executable samples to Microsoft for additional scanning).
I do plan on redoing the Windows Defender tutorial at some point, since in the current tutorial I kinda went overboard excluding every file and folder, but I think it can be simplified to excluding just two folders.