How can I schedule the Disk Defragmenter to run automatically in Windows XP/2003?

When you make frequent writes and deletions from your hard disk your disks will become fragmented with time. Fragmentation of the disks will result in lower I/O performance. In order to prevent loss of performance over time, it is best to perform routine defragmentations of your hard disks.
Manual defragmentation of you disks is possible (see below), but since it tends to take a lot of time, it is best to schedule it to automatically run while you are asleep or away from your computer.
BTW, disk maintenance is also covered in the Schedule Disk Cleanup to Run Automatically in Windows XP/2003 article.
Windows XP/2003 has 2 built-in tools to assist in performing the necessary defragmentations of the disks.

1. Dfrg.msc - The GUI-based Disk Defragmenter MMC is based on the full retail version of Executive Software Diskeeper. This version of Disk Defragmenter MMC that is included with Windows XP has the following limitations:

• It can defragment only local volumes.
• It can defragment only one volume at a time.
• It cannot defragment one volume while it is scanning another.
• It cannot be scheduled. If you need to schedule disk defragmentation, use the Defrag.exe command line tool.
• It can run only one MMC snap-in at a time.

1. Defrag.exe - A Command Line tool. This means you can use the XP/2003 scheduler to automate the defragmenting of your partitions.

Note: The volume must have at least 15% free space for Defrag to completely and adequately defragment it. Defrag uses this space as a sorting area for file fragments. If a volume has less than 15% free space, Defrag only partially defragments it.
Note: You must be a member of the local Administrators group in order to defragment your partitions, however you can run the commands with the RUNAS command:

runas /user:administrator@domain.com "mmc dfrg.msc"

Method 1 - Using Schedule Tasks

1. Open Control Panel, double-click Scheduled Tasks
2. Double-click Add Scheduled Task

1. On the Scheduled Task Wizard dialog, click Next.

1. Click Browse and navigate to windows'system32 folder. Select defrag.exe and click Open.

In the Scheduled Task Wizard dialog, type a name for the scheduled task (for example type Disk Defrag).

1. Under Perform this task, select how often you wish Disk Defragmenter to run. Click Next.

1. Set the time at which you wish the Disk Defragmenter scheduled task to run. This should be a time when your computer is turned on but not under heavy use (i.e. at night time).