Below I have instructions on how to get a Virtual Box image of Chrome
First step to getting Chrome OS working on your Virtual Box install is download a USB or Virtual Box image. Pulled mine from: http://chromeos.hexxeh.net/ or http://chromeos.hexxeh.net/lime.php
Once you have the image extract it to a directory
Now open up Virtual Box and create a new machine. Select the operating system as Linux and the OS is Ubuntu.
I would recommend using at least 1024 MB.
Go through the rest of the screens and start the machine. On first boot up it will ask you to login with your Google account.