The way computer files store themselves in memory is a very "fragmented" process. If you start off with a cleanly installed computer that's only occupying 10% of the total disk space, any new files you install will be stored in successive memory banks. Let's say you keep installing stuff until your disk is about 80% full and now you realize that you should probably clean up your computer more and you go ahead and delete off a bunch of files that you hardly ever use until your disk is only about 50% full. However since you don't know which memory banks these files were in, chances are you've probably deleted bits of data from all over the place, so the next time you want to install something on your computer, anything you install will be stored in bits that are all over the physical harddrive. The computer now has to work harder to piece all of these bits together by jumping around. This can overtime slow down your computer very noticeably.
To correct for this, it's a good idea to defragment your disk drive from time to time so that all the relevant data is once again assembled consecutively to avoid the extra effort, which helps to speed up whatever process you're running. If you're on a Windows platform, you can defragment by clicking Start -> Run -> dfrgui, or it should be under Accessories under System Tools.
Run the program and it will analyze and defragment your computer for you. The best way to do it is overnight when you're a sleep because at times this process can take over 2-6 hours, depending on your CPU speed and the amount of memory you have available and have used. Hope this helps.
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