If you're using Windows Vista, 7, or 8, the operating system automatically defragments itself once a week, late at night. Just search for "disk defragmenter" in the Start menu or Start screen and click the result if you want to see when the process runs. If, however, you're still using Windows XP, you need to manually defragment every couple of weeks or so using the built-in defragmenter (Start > All Programs > Accessories > System Tools > Disk Defragmenter).
However, you'll want to disable Windows' automatic disk defragmenting if your machine contains a solid-state drive. Fragmented data doesn't slow down SSDs because of the way they read and write information--in fact, hardware manufacturers say that defragging adds unnecessary wear that can reduce the life span of an SSD. So in this case, turn it off!
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