It's a 'quirk' of the way Windows saves files to disc... When Windows saves a file, it just looks for the first available piece of free space on the disc - whether the file will fit there or not. It fils the available space with data from the file - then if necessary - looks for more free space. This continues until the file has been successfully saved. This splits (or fragments) a file into multiple pieces - which slows down the access time, as the computer has to search multiple places on the disc to find the data . Using a 'defrag' program 'joins up' all the file fragments into one larger piece of data - then saves it to an appropriately-sized area of free space. Regular disc defragmenting keeps your system running well, and up to speed.