Fragmentation happens when files are divided into pieces scattered around the disk, slowing the computer down. Fragmentation happens when you use your computer frequently, modifying deleting or making files. At some point, the operating system needs to store parts of a file in non-contiguous clusters. This is entirely invisible to users, but it can slow down the speed  because the disk drive must search through the disk to put together a  file. It is a good idea to defrag every couple weeks to improve your speed and longevity. A good time to do it is after you run scan disk. If you find your slack about running it you can set up your task manager to run it regularly for you...I usually have mine set to run while I sleep.
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