Firstly,sorry to just pop in.. I've got some answers to your questions:
You do not need to buy a new computer to try/test Ubuntu or any other operating system. Use virtual machines. There are programs such as VMWare or VirtualBox that always you to run OSes. What I mean, you can install Ubuntu inside a virtual machine and this would not affect your windows. This way you can run multiple OSes, be it Ubuntu, Fedora, OpenSUSE on your windows installation itself.
Ubuntu is a word which means 'humanity to others'. Ubuntu is popular because Linux distros before were not very user-friendly, but Ubuntu implemented somewhat of a GUI. This allowed less 'geek' persons to discover the linux world. And even today, for someone who wants to start to learn Linux, Ubuntu is preferred. Based on Debian, Ubuntu kept evolving to more user friendliness.
Another aspect might be that Ubuntu is free. So no license needed to install, unlike Windows (even though many people used cracked version of Windows)
There are many other things that made Ubuntu popular. It is still gaining ground on Windows.
I prefer Ubuntu because it is very simple to use, and in terms of resources it uses way less than Windows. Moreover, Ubuntu is less prone to viruses than Windows. Security-wise, Ubuntu is good. If a hole is found in Ubuntu, patches are released almost immediately as compared to Windows where patches are released on a time/week basis.
Being open source is another reason I love Ubuntu. There's help everywhere, and I can create my own version of Ubuntu without fearing any legal issues afterwards.
Distros I recommend apart from Ubuntu:
Linux mint - derived from Ubuntu but with more capabilities and apps.
Lubuntu - have same functionality as Ubuntu but uses the LXDE environment which is even more lightweight.
PinguyOS - Based on Ubuntu, but have VLC, and other apps, that people usually install after installing Ubuntu, by default
Fedora
OpenSUSE
Backtrack - Used for penetration and security testing.