Dual booting means having two different operating systems to boot into at startup in the same machine. For the sake of this article, dual booting would refer to having the two Operating Systems- a version of Windows and some flavour of Linux (Ubuntu for this article).
Why dual boot?
Dual booting can serve many advantages, especially when you require the usage of both OS. For instance, if you use softwares like Photoshop and AfterEffects that don't have support in Linux, or if simply you like playing games that are only built for Windows, dual booting is your best bet if you want to use Linux on your machine. If you don't find the use of Windows, you could simply clear all partitions and just have Linux on your machine, and that is completely fine too.
Will it make my PC slow?
Not really. Dual booting would ideally cause little to no change in the performance of a machine.
Note- Most new machines have fast boot enabled with a pre installed Windows. You would need to turn this off in order to dual boot. This would make booting into Windows pretty slow as compared to earlier. However, this would cause little effect in performance after boot.
Change in performance of stand alone Linux compared to a machine with Linux alongside Windows is negligible.
How do I start?
It is recommended to install Linux on top of Windows instead of the other way around. If you don't have Windows installed, you should do a clean install of Windows after removing all other OS. Most of you would already be at this step if you purchased a machine bundled with Windows. If not, you probably want to install Windows from the install disk or a bootable USB. (Read step 2 from Steps to Dual Boot: you'd want to make space for Linux before hand). Once you have only Windows installed you can move on to the next steps.
Steps to Dual Boot
1. Create a bootable disk: To install Linux, you'll need to create a bootable disk. You could either burn a disk or create a bootable USB with this ISO image. Follow these instructions. Go here to download a version of Ubuntu. After downloading you'll end up with an iso file which is essentially the image (an exact copy of the bootable Ubuntu install disk). Now you'd want to get the image burned in a DVD or a USB disk so that you can boot from it. It is advisable to use a USB for this purpose as it can easily be formatted and made reusable. This guide explains how to use Rufus to burn the iso image onto a USB disk. You could also burn the iso image to a DVD as shown here.
2. Make space for Linux: The space available on the hard disk is broken into partitions. These appear as the different drives (C:/, D:/ etc). You'd need to create a partition for Linux to install it alongside Windows.
Note: Two operating systems can not be kept in the same partition.
You could right click on This PC/My Computer and open device manager. Go to disk management and create a new partition by either formatting an existing partition or shrinking an existing one.
Note: Make sure to backup your data if you're formatting a partition. Also remember that there is only a limit to which an existing partition can shrink to.
Alternatively, you can download the EaseUS Partition Master which allows greater flexibility in resizing and other operations.
3. Boot the disk: After you have the bootable disk ready, it's time to boot from that disk. Insert the bootable disk (CD or USB). Power on/Restart machine. Open BIOS settings and change boot order to boot from CD/USB before hard disk. The button to access BIOS settings is different for machines with different make but is usually shown at the first screen when powering on. While you're at the BIOS settings you should disable Secure Boot- This can be found under Boot or in some cases Security category. Just hit Enter when you get the Secure Boot option and then hit disable. Some machines allow a temporary boot order change by letting the user "Select Boot Device" at the startup screen. After the USB/DVD is selected for booting, you'll be taken to the installation menu.
4. Configure and Install: In the main menu select the Install option and you'll be taken to a screen with a few options. Choose "Something Else" as this would allow you to manually configure which partitions you want Ubuntu to use. Select the partition you newly created (in Step 2) as the partition with mount point "/" and file system "Ext4". This will let the installation proceed and further ask for location, keyboard and login information.
5. Done: You're done. The system should reboot taking you to GRUB (short for GNU GRand Unified Bootloader) for the first time which lets you select which OS to boot into. Select Ubuntu and complete your first boot.
Q: What is swap space? Do I need it?
A: Swap space is space on the disk used as if it was temporary memory (like RAM). If you did allocate it, your computer would use it when its RAM is starting to be used up. If you don't allocate it, nothing happens unless you run out of RAM space. Then any program wanting more RAM would be closed. If you want to use hibernate, you need at least as much swap as you have RAM. This is because when your computer hibernates, it copies the RAM to the swap and turns off. To enable swap you need to have a partition of nearly twice the size of your machine's RAM and select it with file system as "Swap" in step 4. Although, it is important to note that newer distros are moving towards needing no swap( Ubuntu 18.04 requires no swap area).
Q: Can't boot into Linux. After installation rebooting takes me to Windows directly.
A: This is mostly caused by Fast/Secure boot enabled for Windows. Disable secure boot in Windows, by going to BIOS settings. Also try to set Windows to boot in Legacy mode.
Q: Can't boot into Windows anymore.
A: This is mostly fixed by performing a boot-repair. To do this, fire up a terminal and type:
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
Hit Enter and then type in
sudo apt-get install -y boot-repair && (boot-repair &)
Wait for the install to complete and from the opened window select the "Recommended Repair" option.
Q: Installation completed but nothing boots. What do I do?
A: Although this could be caused by many different things, try this: Boot from the bootable disk, instead of installing a fresh Ubuntu, select the "Try Linux" option and perform a
boot-repair by launching a terminal inside. Follow the steps in the previous answer.