If you're like me and double as son/IT professional for the family then just tell him the next time it crashed due to viruses obtained through porn it will be bye bye windows and hello Linux.. Just make sure that if you don't live in the same house you set it up so that you can remote connect to it when he gets stuck on something and he can watch what you're doing.. That's why I use VNC on Windows machines, it doesn't take it to the Welcome screen for the remote connection.
As far as using the LiveCD on my desktop machine. I have done that when it crashed once. I think I was using 8.10 though and had it installed and everything and one day it did something weird (and I can't remember what it was) so I just threw Vi$ta back on there.. I may setup 9.04 to dual boot tonight to see how it gets along with all my hardware and dual-monitor setup. I wish they had UltraMon for Ubuntu..