I am straddling two universes at the moment, using Ubuntu 9.10 Karmic Koala and Windows XP on the same machine. I had assumed that I would be rebooting if I needed to grab something from my XP machine that I hadn’t dumped out to a server somewhere. You can imagine my surprise when I went into my Places menu in Ubuntu, clicked on Desktop, and in the new window, saw an icon for my computer right next to my File System.
When I clicked on it, it opened up my Windows XP C drive, and I could browse down to the appropriate folder and open or copy files that I needed to work on. This is a great feature for anyone migrating from Windows to Ubuntu, because you can keep your XP installation alive but also have Ubuntu as your primary environment.
This explains why, when I did the install from the LIVE CD (on a USB drive), Ubuntu imported my Windows XP My Documents folder. No magic involved, it can actually see the files on my Windows partition.