When running a boot-time defrag using Diskeeper 7.x, I get the message the "volume is dirty" and the drive may not be mounted.  I am on a fairly new Inspiron 5100 with Windows XP Home Edition and the c: drive is formatted as NTFS.  I have run this successfully in the past. 

First, does anyone know what might be causing this and second, what can I do to fix it?

Thanks!