Are you talking about a read only that you can undo when you choose, or something that prevents a user from figuring out how to get write access?

The "tohd"/"fromhd" cheatcode copies the CD file system to hd to speed loading and free up the CD drive. It is read only, as to the system files, but you can still write to HD if you enable it. There's also the "bootfrom" cheatcode which allows booting from the ISO file, which is arguably even more secure, but you can still otherwise write to HD. If you don't want people messing with the computer at all, there are some threads floating around about kiosks, and you can find them by typing the word "kiosk" into the search box.

For everyday use, I'd recommend a poor man's install , which will allow you the flexibility of a hd install, if that's what you want, without compromising system files.

jd