If you have lost free space after defragging, then this was almost certainly caused by Shadow Copies.
To delete volume shadow copies manually and safely:
- Start the Disk Cleanup program (normally located in the Accessories folder).
- Select the drive where the shadow volume copies are located (typically, the drive where Windows Vista is installed), and click OK.
- On the More options tab, in the System Restore and Shadow Copies section, click Clean Up.
- Disk Cleanup prompts you to confirm the action. Click Delete.