Several months ago I posted a question about Norton Firewall vs XP'own version. Just for curiosity, I looked up the post, and followed Craven's instructions to access it. To my horror I found that although I had enabled it, back then, it wasn't enabled any more! How or why this happened I don't know, except that perhaps a malign spirit got into my hard drive.
Seriously, I can't figure out what caused this, but I guess I've been lucky, because I haven't had any problems. I think I'll check it out every so often, in case this disablement happens again.