The System Restore process in Windows Vista regularly creates volume shadow copies, which are essentially snapshots of system activity stored on the hard drive.
Volume shadow copies are automatically deleted when:
When either of these conditions happen, the oldest shadow volume copy is deleted.
There is a known issue with volume shadow copies and third-party defragmentation tools such as Defraggler. If there isn't enough room set aside on the hard drive for the volume shadow copies, the defrag process can cause one or more of them to be deleted.
You can delete volume shadow copies manually to free up space and prevent this problem from occurring.
To delete volume shadow copies manually: