Switch to your preferred Unix-like OS (BSD or Solaris is probably better than Linux for this--most people have heard of Linux) but get rid KDE, GNOME or any other friendly desktop environment. Instead, run your preferred window manager (something that looks nothing like Windows or MacOS is better) and use xterms for all your actual interactions. Use lynx as your web browser and mutt as your mail client. (Or if you must use Firefox/Thunderbird, find the most outlandish and confusing theme and set of extensions you can find.)
Then, when somebody asks to use your laptop, just say "Sure, go ahead. This runs $FREE_OS, though, which may not be what you're used to." If they ask for help, tell them which man page they need to read to get the answer.
After three or four iterations of this, nobody will bother you anymore.
(Also, it's a good idea to set up a guest account and some way to to it, just in case you meet somebody who knows Unix. See other posts in this thread for more suggestions.)
I just downloaded it and played with it for a few minutes. Some comments: