Beginner's Guide on Computer Formatting and OS Installation

Recently I helped one of my best friends to format his PC because he was facing continuously decreasing performance issue in his computer. Suddenly an idea came into my mind to write this article from the too many questions being asked by him while we are both working on computer format.
Whenever a new user or beginner thinks of computer format, or wants to format their pc manually, they might have many questions arising in their mind during the process of formatting and installation of new operating system in their PC. Here I would like to explain some highly important things about computer format and OS reinstallation that might be helpful to you.
What does it mean by formatting the computer?
If you have discussed your pc problems with any one and got the answer that you have to format your pc, hard disk or operating system, it means that your computer operating system or more than one of the installed components or software are corrupted, damaged or not behaving as it should.
Formatting pc means erase all the data from your system drive (Hard disk drive where you operating system like windows and Linux is installed) including OS and to reinstall whole OS and all the necessary hardware drivers and software application that you want to use in your pc. In short, it freshens up your whole computer working behavior and performance.
Why you actually need to do this?
Simple, because you want to restore the performance of your pc and remove all OS related issues that you might have been facing. You also have to do this step if your pc and whole OS is infected by computer virus and you need to clean it from the scratch.
What risk is involved, can it be done manually?
According to me, formatting pc is as simple as installing any other application software in your pc if you keep some points cleared in your mind. The main risk involved here is the risk of data loss. Here you must understand the risk of loss of data located on your system drive which may be at the locations listed below.

1. My documents.
2. Desktop
3. My Music
4. My Pictures
5. My Videos
6. System drive (i.e. C:\)

You must backup all the data from above location to make it secure from accident loss of data. Now, the answer to another question that, can it be done manually is yes, you must do it your self. Only you need to do is backup your data and what you must have is bootable disk of your computer operating system.
I have seen many people spending money to format their pc or laptop from the computer shop. It seems difficult only until you don't attempt to do it yourself.