SYMPTOMS

Hard disk can be defragmented using Disk Defragmenter . Performing this task you may get an this error message:
Drive's Contents Have Changed: Restarting...

CAUSE

This problem can take place if a program is executing on the drive when the Disk Defragmenter is running.

RESOLUTION

Clean Boot the Computer A “clean” boot is a technique of reducing problems that happen because of your PC environment. Several problems that happen when you run Windows or other programs can happen because there are clashing drivers, terminate-and-stay-resident programs (TSRs), or other settings that load when you start your PC. Windows XP add a System Configuration Utility tool (Msconfig.exe) to make performing a clean boot easier.

Use Safe Mode

Instead of performing a clean boot, you may restart your PC in Safe mode, and start the Disk Defragmenter tool. After you run Disk Defragmenter, reboot your PC.

To start the PC in safe mode:
1. Click Start, and then click Shut Down.
2. In the Shut Down Windows dialog box, click Restart, and click OK.
3. Press F8 when your PC restarts.
4. In the Windows Advanced Options menu, use arrow keys to choose the proper safe mode option, and press ENTER.
5. If you have a dual-boot or multiple-boot system, select the installation that you need to access using the arrow keys, and then press ENTER.