VirtualBox is a virtualization manager that enables to add different OS on other OS.
Let's admit you have a PC with Windows 7 installed. In the past, if you wanted to have a Linux, you were obliged to install it like any OS with a DVD and boot on it.
And then, reboot each time you wanted to use one of them.
Now, with virtualization, you just have to download a software and while you are still on an OS you can install another one, without rebooting your machine!
And of course, using Windows 7, Mac OS X, any other Linux distribution on your Ubuntu OS! Just with one window of each OS and open them with just one click.
What is really impressive is that you can share files from one OS to other ones.
That's what Oracle does with VirtualBox.
Really important: if you want to avoid a problem when you try to install a new OS on VirtualBox, such like this one:
you'll have to restart your computer and go in the BIOS options to activate the Virtualization.
On my computer, here the way:
When you are in the BIOS, go to OPTION > Virtualization > click ENABLE.
You don't need the TXT mode.
Save and restart the computer.
And if you want a fresh brand new copy, for free, go to the official website: https://www.virtualbox.org/
I've installed VirtualBox on my Ubuntu 64 bit distribution.
For that I downloaded the virtualbox-4.2_4.2.0-80737~Ubuntu~precise_amd64.deb file from the official website.
Indeed, if you have a 64 bit processor, even if you have an Intel processor, you have to take the AMD64 file.
If you have a 32 bit processor, the i386 file is for you.
Once downloaded, just double click it, the Ubuntu Software Manager shows up and ask if you want to install it.
Just say OK.
An other way is to type:
sudo apt-get install virtualbox-4.2
We will be able to install Windows 7, Mac OS X or even another Linux distribution. It's so incredible.
The different steps are quite easy, to be honest.
Of course, you need an .ISO file of the OS you wish to install.
Once VirtualBox is downloaded, launch it by typing in your shell:
On the top, click the New button then Next.
In the Name input, write for example Windows 7 Professional 64 bit.
In the OS Type, select the right operating system and the right version. Then Next.
Select the Base Memory Size allocated to the virtual machine from the host.
It means that this amount will be removed from the host to the guest OS.
If I could recommand an amount, select at least 1024 MB for Windows 7.
One important thing is that you must keep at least 512 MB or more for your Linux to have a stable system.
For the first time, select Start-up Disk and Create new hard disk. Click Next.
Select the VDI (VirtualBox Disk Image) for the file type. Click Next.
For a faster system, I recommand to have a Fixed size for the Storage details. Click Next.
Select at least 25 GB for the Disk file's Size. Click Next (8 Gb will be used for Windows 7).
Wait about 10 minutes for the installation.
Click Create once more to finish the set up.
Your OS is now ready to be installed.
Let's continue by adding the .ISO file to install Windows.
Select the Windows 7 Professional 64 bit virtual on the list, click Settings.
On Storage there is an IDE Controller option.
As a child there is normally Empty written or maybe your .ISO file is already selected.
If there is absolutely nothing, right click IDE Controller and select Add CD/DVD device and click Leave Empty.
Now Empty is added as the IDE Controller child.
Select Empty, on the right, in the Attributes pane, click the CD icon and click Choose a virtual CD/DVD disk file.
Select your .ISO to install.
Then click OK.
Go back and on the list, select Windows 7 Professional 64 bit.
Click it and click the Start green arrow just above.
The installation begins.
The set up is quite the same for Mac and Windows.
Follow the precedent installation of Windows on Ubuntu, and come back here after.
You have change some things in System. Just uncheck the "Enable EFI" option.
OK, you are now ready to click the Start green arrow.
Once done, a black screen will appear.
Type -v in the console to see debug during the installation and press ENTER.
Wait a moment (1 or 2 minutes) and a the language screen will appear.
The Welcome window show up.
You have now to click on the top menu (where there is the apple logo) and click Utilities and select Disk Utility.
Select your OS, for example: 20 GB VBOX HARDDISK.
On the right there is an Erase tab, in the Name input, write a name for your OS.
Then click Erase... just below.
Once done, close the Disk Utility and come back to the Welcome window.
Accept the agreement.
Select your Hard drive and click Continue.
Select what you want, but the minimum is:
Once the installation is completed, there will be a countdown and the OS will restart.
When it restarts. Stop this Virtual machine. In the Settings of VirtualBox, go to Storage and remove the MacOS.iso.
Then click the Start green arrow to start your Mac OS X.
A new screen will appear to Select your Region, your keyboard, if you already own a Mac, enter your Apple ID, Registratin information, Create your account, select Time Zone.
Finally click Done.
Well done, you made it!