Wubi

From the site Wubi
Wubi is an unofficial Ubuntu installer for Windows users that will bring you into the Linux world with a single click. Wubi allows you to install and uninstall Ubuntu as any other application. If you heard about Linux and Ubuntu, if you wanted to try them but you were afraid, this is for you.

Ubuntu is an operating system that includes a lot of free and open source applications and uses Linux as its core. Linux is like this amazing solar-powered engine that can be used in a street car, in a F1 or it can be daisy chained to drive a truck or an airplane. Ubuntu is like a car using the Linux engine, a zero emission, fully accessorised and easy to drive all-terrain, with power, acceleration and looks matching far more expensive supercars... Imagine something like that... ...for free.