Sometimes you need to perform certain tasks that are only doable on a specific operating system, like running a particular program for example. One of the ways to do it is through Dual Boot, which basically installs two (or possibly more) operating systems on your computer. You then get to choose which operating system to run when the computer starts.
When Dual Boot is installed, you will be prompted to select the operating system you wish to use every time you turn on your machine. So switching from an operating system to the other will require you to restart your machine.
Your hard drive is divided into partitions, typically one for every operating system installed on it (if you happen to have more than one). How does the computer know where the operating system is? In fact, you could have multiple hard drives or other storage media. Your computer needs to know where exactly to find the operating system kernel in order to run it.
Let us see how booting works, so that we can understand what Dual Boot does.
The booting process starts when you turn on your machine. The first thing that happens is that there is a small program built onto your motherboard called a BIOS (Basic Input/Output System). The BIOS can be configured to launch the next step from a particular storage media, typically the hard drive (this is how you install/format your operating system - you tell the BIOS to launch from a CD (or some other device) and this CD will install the operating system on some storage device such as your hard drive).
Once the BIOS decides which storage device to launch from, it will go to a section called the MBR (Master Boot Record). The MBR is located at the first portion of a partition of a storage device. The MBR is specific to one (or more) operating system(s), so it knows where the operating systems' kernels are located on the storage device and can thus jump to them and run them. The kernels are essentially the cores of operating systems, so once control has been passed to a kernel, the operating system takes control from that point on.
As mentioned earlier, an MBR is specific to an operating system. MBRs contain information about the data in the partition, and most importantly, some code that knows how to launch the kernel of that particular operating system (e.g. Windows). This piece of code is called a bootloader or a bootstrap. Since the code is specific to a particular operating system, it cannot possibly run, or point to more than one.
However, we don't have to be bounded by the default bootloader that is given when we install Windows or Linux. In fact, there is a very popular bootloader called GRUB (which we will show how to install). This bootloader can be configured to prompt the user and launch an operating system from a list of pre-defined operating systems available on the machine. The idea is that this initial bootloader will give execution to the actual, default bootloader of an installed operating system as a second stage so that we can pick an operating system instead of running the only installed one.
Some people prefer certain operating systems more than others, so they tend to use one primary operating system for daily work. However, some functionalities may only be available on one operating system, different than one might use in his daily life. For that reason, Dual Boot is helpful to allow users to have the best of both worlds, or both operating systems.
For example, some people highly prefer Linux over Windows. But there are not many games available for Linux users. So Dual Booting Windows with Linux is a good option in that case since one can do all his work on Linux, and switch to Windows when he decides he wants to play games.
There are many cases in which Dual Boot may be useful. Sometimes it is just done to experiment and try new operating systems. Other times it is necessary to perform daily tasks.
The instructions below are mainly aimed at Linux Ubuntu installation from an existing Windows operating system installed. However, the differences between Ubuntu and other Linux distributions are subtle and we will provide brief information about how to install other distributions.
To install dual boot, you will need the following:
Linux has many distributions. Each one has pros and cons. You may want to check out their websites, or see some videos to see which one you may need.
We will mainly show how Ubuntu is installed, which can be obtained from their website here. You can get either the 64 or the 32 bit version.
To install a different distribution, go to their main website and you should find a download link (depending on the distribution).
This step requires shutting down your computer. Make sure you have access to this page from another device, or print the document.
Installation steps depend on the Linux distribution you chose. The following instructions assume Ubuntu. If you chose a different distribution, go here.
Congratulations! You have successfully installed Dual Boot!
Every time your computer starts, you will be prompted (by GRUB) to select an operating system. You should see Windows and Ubuntu (or whichever distribution you selected). You can select either and your computer will launch the selected operating system.
You don't need to repair anything, avoid repairing and try to run Windows again. Windows is basically complaining that you installed GRUB overriding its' default bootloader.
You need to have a Windows CD or ISO image. You should install Windows first and then go here. Installing Windows on top of Linux is much more difficult than installing Linux on top of Windows. So it is easiest if you install Windows first.