Continuing you on from part two:
If the user is working with XP, I would say that the chances are pretty good that this could be done pretty easily. However with Vista, I have heard mostly of problems due to how Vista does it MBR to some having success. Frankly, in my honest opinion, dual-booting is stupid.
If you want to use Ubuntu, but still rely on Windows for whatever reason, use Wubi. It will allow Vista to remain master of the Universe like it prefers to be in the MBR realm and you can still “install” Ubuntu.
What else should someone making the switch to Ubuntu know about? I guess is depends on what you are using your computer for. Generally speaking, the biggest challenges are simply getting used to the different desktop options (KDE, GNOME, etc).
For the most part, I would start off by heading to Amazon and picking up a book called Ubuntu for Non–Geeks. Also, realize there is a whole world of Linux outside of Ubuntu. PCLinuxOS, Linux Mint, Simply Mepis are all good options for you to explore.