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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 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, though, 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. 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. Eventually, you wind up griping to your buddies or spouse that you need a new computer, because this one seems to have gotten so slow.
The solution: Disk Defragmenter, a program that puts together pieces of files that have become fragmented (split into pieces) on your drive. The "defragger" also rearranges the files on your drives to make the operating system and programs load more quickly. A freshly defragged PC feels faster and more responsive than a heavily fragmented one.
The big news in Windows Vista is that its disk-defragging software runs automatically at regular intervals, in the tiny moments when you're not actually typing or clicking. It's like having someone take out your garbage for you whenever the can is full. Slow-PC syndrome should, therefore, be a much less frequent occurrence.
Defragging Settings

Even though Vista defrags your hard drive automatically in the background, you can still exert some control. For example, you can change the schedule, and you can trigger a defragmentation manually when you're feeling like a control freak.
Start by opening the Disk Defragmenter main screen, using any of these techniques:

• Choose Start Control Panel. Click System and Maintenance"Defragment your hard drive." (That last link is at the bottom of the window, under the Administrative Tools heading.)
• Right-click the icon of a hard drive in Windows Explorer; from the shortcut menu, choose Properties. In the Properties window, click the Tools tab, and then click Defragment Now.
  Note: The button may say Defragment Now, but it won't actually start defragmenting when you click the button. You'll first pay a visit to the main Disk Defragmenter screen.
• In the Start menu's search box, or at a command prompt, type dfrgui.exe and press Enter.
  Tip: Throughout Windows, and throughout its book and magazine literature, disks are referred to as volumes. Usually, volume means disk. But technically, it refers to anything with its own disk icon in the Computer windowincluding disk partitions (page 601), DVDs, and so on.
• Disk Defragmenter isn't the only tool for the defrag job; the world is full of disk-defragmenting programs that offer additional features.
  For example, some of them track how often you use the various files on your drive, so they can store the most frequently used files at the beginning of the disk for quicker access. In some programs, you can even choose which files go at the beginning of the disk.
  Do these additional features actually produce a measurable improvement over Windows Vista's built-in defragger? That's hard to say, especially when you remember the biggest advantage of Disk Defragmenterit's free.
• Adjust the schedule. Click Modify Schedule. A screen appears, showing that Vista ordinarily defrags your disk late every Tuesday night (1:00 a.m., in fact). You can use the pop-up menus here to specify a Weekly, Daily, or Monthly schedule, complete with day-of-week and time-of-day options. Click OK and then OK again.
• Manually. Click Defragment Now; the defragmenter does its work. Depending on the size of your hard disk, your processor speed, and the amount of fragmentation, it will take anywhere from several minutes to several hours.