This one is hard! and you can give 512MB of RAM to Chrome OSOr...if that doesn't work.
Download the link and extract it, now place it somewhere simple, like your Desktop! Go in VirtualBox and click "New"
Name it whatever, and then put the OS type as "Linux" and then at version put "other Linux"
Now, put your RAM to 1 GB (or 1024 MB)
In the Hard Drive section click "use an existing Hard Drive" and find the file you extracted earlier!
Congrats! It's installed.
Now to make the Hardware work;
Click the Network tab and expand the Advanced section. Click the Adapter Type drop-down box and select Intel PRO/1000 MT Desktop (82540EM) in the list.
Mouse: When you start Chrome OS, the mouse won’t work. To make it work, click the Machine menu and select the Disable Mouse Integration option.
You can now click inside the virtual machine’s window to make it grab your mouse cursor. To release the mouse cursor from the virtual machine, press the right Ctrl key. (If you’ve set a different “host key” instead of right Ctrl, it will appear at the bottom-right corner of your virtual machine window.)
Chrome OS will take you through a few screens of setup (press Esc to skip the update check when prompted.) You’ll then be able to sign in with your Google account. If you use the Chrome browser, all your Chrome apps, extensions, and settings will be synced if you sign in. You can also click Browse as Guest to try Chrome OS without signing in. Of course, one of the big advantages of a Chromebook is how well it syncs with your Google account and integrates with Google’s web services, and you’ll lose that if you opt not to sign in.
Chrome OS will start up as if you’d just purchased a Chromebook, although the performance on a real Chromebook will be better than what you’ll get in a virtual machine.
Congratulations! You've installed Chrome OS in Virtual Box!