Kubuntu = Ubuntu with KDE as its desktop environment instead of Gnome. KDE is more like Windows while Gnome is more like Mac OS (although you can install multiple desktop environments and choose from them later). Also, KDE comes with a clipboard applet, something you have to install under Gnome. Ubuntu uses apt to manage your software installations (you can also add extra software repositories to your lists), and you can easily install synaptic and a bunch of other programs from there. Also, by default it is secure and its security is set up correctly.
You can go to
http://www.ubuntuguide.org and to learn how to set up the machine to run divx and dvds and all sorts of things.
I recommend installing OpenOffice, Abiword, The Gimp, K3B, Firefox, acroread, gnochm, PAN, Evolution, GAIM, xmms, xmms-mp3, streamtuner, f-spot, gwenview, and xine for starters.
SuSE is good too, but I find it to be annoying to maintain. Fedora Core 4 is the freeware version of Red Hat, but 4 just came out and doesn't have a ton of software support unless you have a 32 bit intel compatible processor (64 bit is nice, but lacks widespread support). I hear Mandriva is good, but haven't used Mandrake since version 5.