Chromium OS is the open source version of Chrome OS. Chromium OS isn't as polished as Chrome OS and isn't guaranteed to run properly on your hardware compared to buying a Chromebook.
Getting & installing Chromium OS
I Googled Installing Chrome OS Onto A Laptop and came across this handy little tutorial that also then gave me links to where to get the software required to install Chromium OS.
I then downloaded the latest build of Chromium OS from the 20th of April 2013 from here and followed the instructions for Windows and downloaded Image Writer for Windows from here and wrote the Chromium OS image to a 4GB Sandisk USB stick.
I then downloaded Plop Boot Manager from here and burnt it to a blank CD if I was unable to boot from USB.
Installing Chromium OS
The first thing that I did was boot my Laptop from USB and try out Chromium OS without installing to make sure that the WiFi Card worked. I then re-booted back to Windows and did a backup of my pictures and downloaded files.
I then booted back into Chromium OS from USB and once logged in I:
pressed Ctrl+Alt+T to bring up the command prompt
Typed install followed by pressing Enter
when asked for a password i entered facepunch
Chromium OS then Installed, I then followed the prompt to re-boot and remove the USB Stick.
Setting up Chromium OS
I then selected my language, Keyboard layout & WiFi Network, entered my WiFi password and was presented with a standard Google Login/Signup prompt. I then logged into my Google Account, set the correct Time zone and waited for all my Chrome apps & bookmarks to sync.
Chromium OS is a lightweight OS compared to the likes of Windows, Linux & Mac OSX, the 1TB drive in my Laptop has 908.2GB of free space remaining compared to the just over 800GB of free space that I'd got free when I was running Windows or Linux.