When I try to do a delete on a .mpp file it gives an error message "The file is currently checked out or locked for editing by another user". The file doesn't seem to be checked out. I am not sure about file locking.
Even though the file doesn’t show as being checked out, it could still be open by another user and that would stop you from doing anything to it. First, I would check to make sure no one is working on that file.
http://www.mycodekb.com/CodeDetails.asp?CodeID=584