 When a computer system is powered on, it goes through this process we call booting. The first thing that happens after the power is turned on is that the CPU executes instructions in a chip on the motherboard called the BIOS. The BIOS is simply a chip which stores a small amount of code and the CPU looks there to find its first instructions. What this code in the BIOS then has the CPU do is look for an operating system on one of the storage drives in the system. It's likely a hard drive. When an operating system is found, it is then loaded into memory and the CPU starts executing that operating system code. Now after powering on the system, the first thing you should see is this screen drawn by the BIOS. And typically there's a message at the bottom or at the top that tells you you can hit a certain key on the keyboard like say F2 in this example. And that will take you to the BIOS's configuration menu where you can configure some basic options in your system. Most importantly, you can tell the BIOS which drives it should look at when it tries to find an operating system and very importantly tell it which devices it should look at first. Because you might have different operating systems installed on different drives and you want to be able to control which operating system actually gets loaded. So you may be wondering what if I don't have an operating system installed on any of my storage drives. Well, this for example is how you would install Windows. You would insert the Windows install disk, a DVD usually, into your DVD drive and then you would boot from that DVD drive, meaning you would make sure in your BIOS that it is configured to try and find an operating system first from your DVD drive. Because the installation disk contains sort of like a mini operating system which is just for the purpose of installing Windows. So your system boots from that DVD, the Windows installer loads and then it asks you to enter a few settings like specify which keyboard you want to use and a few other questions like most importantly which drive do you want to install Windows on. And then once you're done entering these settings, the actual installation takes place, all the necessary files are copied onto the destination drive. And then once that's done, your system will restart and it's now ready to boot from the drive onto which you installed Windows. So if you boot the system from that drive, Windows will load for the first time. Maybe it has to do a bit more business to finish the installation process, but after that's done then you'll be ready to go. So as long as nothing goes wrong, the whole process is really fairly simple and it's usually no more complicated these days to install other operating systems like say Linux. Now the operating system is ultimately the piece of code responsible for talking to all of the particular hardware devices in your system. Programs themselves like say Microsoft Word do not directly talk to like your hard drive. What's really going on there is that Microsoft Word when it say wants to write a file is it makes a request to the operating system to Windows. It says I want to write this file and then Windows is responsible for actually writing the file onto the disk. So all the rights and responsibilities of controlling the hardware is left up to the operating system. One problem however is that there's an enormous variety of different hardware devices out there and to control these devices the operating system has to speak to each device in its own particular way. So for example the way an operating system needs to talk to one make a video card is not the same as it needs to talk to some other make a video card. So the code which an operating system needs to use to talk to a particular device has to be unique for that device. And there are so many devices out there it's not actually practical for say Microsoft to be responsible for supporting all of those different devices. And it's very often left up to the manufacturer of a device to create that code. And these pieces of code which control particular devices are called drivers. You should think of these drivers as like part of the operating system but independent modules that get plugged in. And so say when Windows is installed on your machine one of the most important things it does in that installation process is trying to detect what hardware devices you have and make sure that all the drivers are there for those devices. The problem with this is that new devices are being released all the time and the drivers for existing devices often get updated so as to fix bugs. So even once Windows is installed and running we still need the ability to upgrade and install new drivers. And this can typically be done just like installing a program. Device manufacturers make their latest drivers available on the internet so users can download and install them.