I would like to create a application that would act like, or possibly replace, Windows Explorer where my clients could "check-out" the project folders.

Now I have a brief understanding of how to do this but I have no clue how to LOCK these folders so that they cannot update the file while someone has it checked out. My problem is that sometimes we lose data because someone "checked-out" a project to go to a client meeting, but while they were gone, someone else went into the data and changed/added/deleted as needed. When that person got bac and uploaded the folder back into the server, we lose any essential data that was made to it.

Hope I explained it enough. Any thoughts on locking the folder down with permissions ONLY to the program that acts liek a librarian?