Linux comes in many different incarnations, and Ubuntu is only one of these. Arguably the easiest for Linux beginners to get the hang of. Basic apps like Firefox, Openoffice, and (i think) VLC media player come preinstalled, and there is a pretty decent UI right off the bat. And as in most versions of Linux, the tweakability is nearly endless.
Windows programs will not work in a Linux OS without some kind of virtualization software.
I use Ubuntu (Linux), Mac OSX, and Windows XP. I tried Windows Vista and hated it, so I switched back to Windows XP for a few programs I like that can only be run on Windows (like Adobe Audition). I prefer Mac OSX because its more secure and doesn't require ongoing maintenance like Windows does (Defragmenting, cleaning the registry, etc).
Ubuntu is going to be the easiest for beginners to get up and running. It does typically work straight away on most systems, but it's possible you may have to hunt for some drivers. If you have a big-name brand of PC such as HP or Gateway, it'll probably work just fine.