The backup was not successful. The error is: There is not enough space on this drive to save the backup. Free up space by deleting older backups and unnecessary data or change your backup settings. (0x81000005).
This message is generated when Windows cannot perform a backup due to a lack of disk space.
Windows 7 does not automatically delete old backups, and as such you will need to manually delete old backups in order to make room on the disk where your backups are stored.
The easiest way to do this is to click the notification in the tray icon that will be produced when the backup fails, alternatively you can also attempt to run the backup manually (even though it will immediately fail) and click on the "Manage Space" link, which allows you to delete old backups.