badlad: that's all very well, but Mandriva isn't as popular as it used to be (and you want to use the most popular as a beginner, since you get the most support), and it's designed for more powerful machines.
The difference between Ubuntu and Xubuntu is GNOME to XFCE.
These are 'desktop environments' --- basically, the programs that display windows and icons and stuff on screen, and some programs to change settings, things like that. GNOME is good, but XFCE is specifically designed to be similar, but more lightweight, making it better for lower powered machines.