Linux is the kernel which all Linux distros. (including Ubuntu) are built upon
http://en.wikipedia.org/wiki/Linux_kernel
What is Ubuntu
Ubuntu is a community developed operating system that is perfect for laptops, desktops and servers. Whether you use it at home, at school or at work Ubuntu contains all the applications you'll ever need, from word processing and email applications, to web server software and programming tools.
Ubuntu is and always will be free of charge. You do not pay any licensing fees. You can download, use and share Ubuntu with your friends, family, school or business for absolutely nothing.
We issue a new desktop and server release every six months. That means you'll always have the latest and greatest applications that the open source world has to offer.
Ubuntu is designed with security in mind. You get free security updates for at least 18 months on the desktop and server. With the Long Term Support (LTS) version you get three years support on the desktop, and five years on the server. There is no extra fee for the LTS version, we make our very best work available to everyone on the same free terms. Upgrades to new versions of Ubuntu are and always will be free of charge
http://www.ubuntu.com/products/WhatIsUbuntu
Installing Ubuntu as a dual-boot with Windows without partitioning
https://help.ubuntu.com/community/Wubi
You keep Windows as it is, Wubi only adds an extra option to boot into Ubuntu. Wubi does not require you to modify the partitions of your PC, or to use a different bootloader, and does not install special drivers. It works just like any other application.
Wubi keeps most of the files in one folder, and if you do not like it, you can simply uninstall it as any other application.
Boot in to windows insert the Ubuntu 10.04 LiveCD and you will offered the option of installing inside windows which is where the Wubi installer comes in, you will be asked how many gigabytes you wish to allocate to Ubuntu (I recommend 8gb) then you set a password for your installation then click install and thats it.
Once Ubuntu is fully installed upon starting your PC you will be given a choice of which operating system you want to use Windows or Ubuntu
Ubuntu 10.04 Download
http://www.ubuntu.com/getubuntu/download
Ubuntu 10.04 (Lucid Lynx) User Guide
http://ubuntuguide.org/wiki/Ubuntu:Lucid
Linux Mint 9 (Release Candidate) has a similar feature called mint4win and the directions given above for Wubi can be followed
http://duncsweb.com/2009/09/27/mint4win-a-wubi-based-installer-of-linux-mint/
Linux Mint 9 (Release Candidate) Download
http://www.linuxmint.com/edition.php?id=52
Linux Mint 8 Helena User Guide (also applies to Linux Mint 9)
http://www.scribd.com/doc/23787138/Linux-Mint-8-Helena-User-Guide
Ubuntu 10.04 and Linux Mint 9 can also be run straight from the LiveCD without touching your Hard drive
LUg.