I don't know why a folder of mp3s would be marked a system file. Maybe there are thumbdb files in there causing the problem. If you have viewing of hidden files turned on (Windows Explorer > View tab > tick Hidden items), then you could have a look.
But the easiest fix would just be to shut down MC, then rename the problem folder(s), create new folder and name it the same as the original(s), then copy all your mp3s over to the new folder(s). Restart MC and import those folders. If all works as expected, delete the old folder(s).
Of course, I'm assuming that the folders aren't in fact System Folders. If you have allowed MC to scan and import files from all of your PC, which it does by default in a new library unless you stop it, then the above advice could be dangerous, and break your Windows installation. But if the problem folders are those you have created yourself at some time in the past, then all should be well.
