Check if the Disk Defragmenter service is running.
a. Click Start and type
services.msc in the search box.
b. On the
Services window, look for
Disk Defragmenter service and Click OK.
c. Check if the status of the Disk Defragmenter service is set to Automatic.
If it is, then a program running in the background could be causing it, try this troubleshooter;
http://support.microsoft.com/kb/929135