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Defragment the HDD (Hard Disk drive)
One of the reasons why computer is slow, may be the amount of fragmented files. When you install and uninstall applications or when you move files, you leave behind leftovers known as fragmented files. Fragmented files can slow your computer down.
Windows has a easy-to-use built-in utility that defragments your hard drive called Disk Defragmenter (type Disk Defragmenter in the start menu search box or go to Start -> All programs -> Accessories -> System tools -> Disk Defragmenter -> right-click it and Run as Administrator), which helps you defragment your computer's HDD.
All you need to do is to select the partition you want to defragment ("C:" ; "D:" and etc.), click on Defragment Disk and wait until the defragmenting process is completed. Make sure to defragment all the drives on your computer ( "C:" "D:" "E:" and etc.) at least once per month.
If you wish, you can set it to defragment your computer automatically. Just click on Configure Schedule (in the Disk Defragmenter window) and pick a time (when your computer is always turned on).
Note: If you are using SSD (Solid-state Drive) DO NOT DEFRAGMENT IT, because if you do, you might damage it. SSDs store data in a sequential order (as opposed to random order).
Also, don't defragment the System Reserved partition, because it is meant only for system files, which are important for the computer.