When you save a new file, Windows records its information onto the hard drive in small pieces called blocks. On a new PC, Windows lays the blocks end-to-end on the hard drive surface. Later, when you type more data into a document (thus enlarging it), the file no longer fits in the same space. Windows XP puts as much of the file in the original location as can fit, but may have to store a few of its blocks in the next empty spot on the hard drive.
Ordinarily, you’ll never even notice that your files are getting chopped up in this way, since they open promptly and seamlessly. Windows keeps track of where it has stored the various pieces, and reconstitutes them when necessary.
As your drive fills up, the free space that’s left is made up of smaller and smaller groups of blocks. Eventually, a new file may not fit in a single “parking place” on the hard drive surface, since there are no free spaces left that are large enough to hold it. In fact, Windows may have to store a file in several different areas of the disk, or even hundreds.
When you later try to open such a fragmented file, the drive heads (which read the disk) must scamper all over the disk surface, rounding up each block in turn, which is slower than reading contiguous blocks one after the other. Over time, this file fragmentation gets worse and worse and may eventually result in noticeable slowdowns when you open or save files.
The solution: Disk Defragmenter, a program that puts together pieces of files ...