How to defragment hard disk on Windows?

As time goes by, files are written to, stored and removed from hard disk leading to Fragmentation - meaning some files no longer occupy contagious space as a single unit. Instead, files are split into parts and occupy wherever there is space available. Fragmentation leads to slower computer due to increased work on the hard disk head. In this post, we shall see how to analyze defragmentation and perform defragmentation in Windows manually.