Re-purpose Your Old Laptop to Make Your Own ChromeBook

1) Now, let’s go to Built by Hexxeh to download the latest ChromiumOS Vanilla build. The download size is around 300MB, and it’s wrapped up in a ZIP format.

2) Once you have it downloaded, extract the IMAGE file from the zip file to a convenient location, i.e. your download folder.

3) To load it up to the USB drive (must be minimum of 4GB), you will need a tool called Windows Image Writer, which is a free portable tool that writes IMG file directly to a USB drive and makes it bootable as well. Make sure back up your USB drive first, as it will be reformatted to a format Windows doesn't recognize.

Before you download, BE AWARE of a few things. These builds:
Are totally unofficial Chromium OS builds and do not represent the full Chrome OS experience.
Are offered without any support whatsoever, you use them at your own risk.
Do NOT automatically update, you'll need to download a new build regularly if you wish to update.
Do NOT include Flash, Java or the Google Voice/Video plugin for licensing reasons.
May simply not boot on your hardware: not all configurations work!

From: http://www.nextofwindows.com/re-purpose-your-old-laptop-to-make-your-own-chromebook/