Installing Ubuntu inside Windows using VirtualBox

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The screenshots in this tutorial use Ubuntu 12.04, but the same principles apply also to Ubuntu 12.10, 11.10, 10.04, and any future version of Ubuntu. Actually, you can install pretty much any Linux distribution this way.
Introduction
VirtualBox allows you to run an entire operating system inside another operating system. Please be aware that you should have a minimum of 512 MB of RAM. 1 GB of RAM or more is recommended.
Comparison to Dual-Boot
Many websites (including the one you're reading) have tutorials on setting up dual-boots between Windows and Ubuntu. A dual-boot allows you, at boot time, to decide which operating system you want to use. Installing Ubuntu on a virtual machine inside of Windows has a lot advantages over a dual-boot (but also a few disadvantages).
Advantages of virtual installation

• The size of the installation doesn't have to be predetermined. It can be a dynamically resized virtual hard drive.
• You do not need to reboot in order to switch between Ubuntu and Windows.
• The virtual machine will use your Windows internet connection, so you don't have to worry about Ubuntu not detecting your wireless card, if you have one.
• The virtual machine will set up its own video configuration, so you don't have to worry about installing proprietary graphics drivers to get a reasonable screen resolution.
• You always have Windows to fall back on in case there are any problems. All you have to do is press the right Control key instead of rebooting your entire computer.
• For troubleshooting purposes, you can easily take screenshots of any part of Ubuntu (including the boot menu or the login screen).
• It's low commitment. If you later decide you don't like Ubuntu, all you have to do is delete the virtual hard drive and uninstall VirtualBox.

Disadvantages of virtual installation

• In order to get any kind of decent performance, you need at least 512 MB of RAM, because you are running an entire operating system (Ubuntu) inside another entire operating system (Windows). The more memory, the better. I would recommend at least 1 GB of RAM.
• Even though the low commitment factor can seem like an advantage at first, if you later decide you want to switch to Ubuntu and ditch Windows completely, you cannot simply delete your Windows partition. You would have to find some way to migrate out your settings from the virtual machine and then install Ubuntu over Windows outside the virtual machine.
• Every time you want to use Ubuntu, you have to wait for two boot times (the time it takes to boot Windows, and then the time it takes to boot Ubuntu within Windows).

Installation Process
The first thing you have to do is obtain VirtualBox. Visit the VirtualBox website's download page. Install it the same way you would any normal Windows program. Follow these instructions to get a Ubuntu disk image (.iso file).

After you launch VirtualBox from the Windows Start menu, click on New to create a new virtual machine. When the New Virtual Machine Wizard appears, click Next.

You can call the machine whatever you want. If you're installing Ubuntu, it makes sense to call it Ubuntu, I guess. You should also specify that the operating system is Linux.

VirtualBox will try to guess how much of your memory (or RAM) to allocate for the virtual machine. If you have 1 GB or less of RAM, I would advise you stick with the recommendation. If, however, you have over 1 GB, about a quarter your RAM or less should be fine. For example, if you have 2 GB of RAM, 512 MB is fine to allocate. If you have 4 GB of RAM, 1 GB is fine to allocate. If you have no idea what RAM is or how much of it you have, just go with the default.
Click Next.

If this is your first time using VirtualBox (which it probably is if you need a tutorial on how to use it), then you do want to Create new hard disk and then click Next.

Click Next again.

Theoretically, a dynamically expanding virtual hard drive is best, because it'll take up only what you actually use. I have come upon weird situations, though, when installing new software in a virtualized Ubuntu, in which the virtual hard drive just fills up instead of expanding. So I would actually recommend picking a Fixed-size storage.

Ubuntu's default installation is less than 3 GB. If you plan on adding software or downloading large files in your virtualized Ubuntu, you should tack on some buffer.

Click Create and wait for the virtual hard drive to be created. This is actually just a very large file that lives inside of your Windows installation.

The next thing to do to make the (currently blank) virtual hard drive useful is to add the downloaded Ubuntu disk image (the .iso) boot on your virtual machine. Click on Settings and Storage. Then, under CD/DVD Device, next to Empty, you'll see a little folder icon. Click that.

Select the Ubuntu .iso you downloaded earlier.

Once you've selected it, click OK.
Then double-click your virtual machine to start it up.

You may get a bunch of random warnings/instructions about how to operate the guest operating system within VirtualBox. Read those, and then you may also want to mark not to see those again.

Wait for Ubuntu to boot up.

Once it's started up, just follow the regular installation procedure as if you were installing Ubuntu on a real hard drive (instead of a virtual one).

Afterwards, in order to use your virtualized installation (instead of continually booting the live CD), double-check that the CD/DVD Device entry is Empty again.