Hi, thanks for the good description!
Vista, okay... unfortunately I don't have a machine with Vista to test it myself. From what I've read about it on the net, I believe the problem might come from the inaccessibility of certain directories under Vista.
L++ tries to access files inside its own subdirectories when it loads the sprites, levels or configuration files, or when it writes changes to them. network/, orig/ and test/ are subdirectories of (lpp's own folder)/levels/. This approach was clean under older versions of Windows -- one could uninstall the game by removing its directory, and everything was gone. It works under Unix variants as well, even if it's not the standard behavior there as far as I know.
I haven't read enough of how Vista handles these situations to give a good answer. I will think about a future-proof solution when this semester's exams are over in some weeks. Until then, maybe it works to put a copy of the L++ folder directly into the Vista virtual store?
-- Simon