You could automate this a FireFox extention called x that will clear everything at once
http://texturizer.net/firefox/extensions/#x (suggested in another thread).
This shouldn't be a problem if you're using XP, fast user switching with multiple accounts should make it so they won't be able to view anything personal. Just set them up with a guest account instead of giving them priviledges to yours.
BTW, clearing history for IE was covered on the thread
http://www.tfproject.org/tfp/showthr...irefox+history