Well #1 I would setup a backup such as Carbonite so if something happens you can roll back. There isn't really a locking feature in Windows. You could do one of the following
Remove all but read access to the directory. (Members of the administrators group would have to consciously re-add write access)
Get software such as deep freeze. You mark directories as frozen. Even if you delete the folder a simple reboot will roll it back. When you want to change the directory you put the directory in thaw mode, change and refreeze