full Debian Sarge setup with X, kde, openoffice, gaim...
the advantage is that apt-get update/upgade can be set as a cronjob and be steered to security patches...
another benefit would be propper security as far as user managment... goes, in knoppix you just do sudo su to become root/admin.
here is a posting wiht more on locking down knoppix.
http://www.knoppix.net/forum/viewtopic.php?t=9905

there in the begining i talk about instaling bare minimum, that is still valid.
just to give an hint as to what is needed, i have a 122Mb install that i frequently use to mail. ftp, surf the web, instant messaging (ICQ,MSN...).
granted it is not pretty (console only). but it works and gets the job done.

if you skip KDE and GNOME (gaim is possible to get working without gnome installed)... use icewm instead for example and install bare esentials and be strict in securing the install not much should be possible to wreck.
but as there are ALWAYS ways around security... you should plan for making speedy recoveries.

you could use dd to make a image of root-partition of the install (dd if=/dev/hdXY of=/home/user/backup.img)
If you make the / partition 600Mb (you should be able to fit everything needed on 500M or so so) and then include a backup copy of her /home/user partition (separate partition is to prefer as you then can wipe the / partition and she stil has her files... intact) just be certain to do it AFTER after the setup of gaim, mailaccount... (to preserve the settings).
all the config files in her /home/user and the image of entire / should fit one 700Mb CD with plenty space over.
now it should take a VERY short time to restore computer to where it was. just dd the image back over the / partition (dd if=/mnt/cdrom/backup.img of=/dev/hdXY) and copy the configfiles to her /home/user (only if you have removed them or she is having trouble), next you do a apt-get update&&apt-get upgrade and you are set.
but as i said read more on the link and keep asking.