Ubuntu is a Linux distribution based on Debian that is gaining a lot of popularity in the desktop OS.
This distribution focuses on ease of use and comes bundled with tons of open source software like OpenOffice, The Gimp, and the Evolution mail client with countless more available in the package manager. Ubuntu comes on a LiveCD which allows you to test out the OS without actually installing it, if you like it you can just install it straight from the CD. Partitioning is done with the Gnome Partition Editor and is rather painless.
The nice thing about Ubunt is that everything just works, and in the rare event there is a problem, there is a huge forum community that has the resources to solve almost anything you search for. I’ve been a windows user for longer than I care to remember, but since I installed Ubuntu, I haven’t been back once. I would highly recommend at least trying out the LiveCD, it’s worth it.