1. Laptop: Ubuntu 8.10 Desktop: Windows XP SP3
2. Laptop: Because it works, and its easy to setup. Desktop: Because I need a windows box and I like to play games.
3. Laptop: Adv. Apt-get is the most awesome thing in the world. It just works.

Dis. Barely any. Most of the problems I did have are fixed. Biggest draw back is not having MS Office (yes I know it works in wine). Desktop: Adv. Its still running barely. Dis. It completely waste the power of my quad-core.
4.
www.ubuntu.com
http://ubuntuguide.org/wiki/Ubuntu:Intrepid
5. It should be gnome or KDE (I like gnome). Really I do like the windows interface.
On the Fedora switch, I went over to Ubuntu around version 6, never looked back. I like it, it really all depends on you.