It is prompting you to perform the scheduled "incremental" backup. After the initial backup, all subsequent backups will back up the new and changed files. If you've turned on the computer and used email, for example, you have new and changed files. If nothing much has changed, the backup should not take 7 discs or 2 hours. If you don't wish to backup at all, or be prompted for backup, but run a manual backup as desired, (and assume the risk that a problem happens long after an updated backup has been performed) you can turn off the scheduled backup.
Open OneCare
Click Change Settings
backup tab
Click change settings in the lower left
Make sure that the first screen for Centralized Backup has "No Centralized backup" selected
Click your PC on the left.
Uncheck the selection for "Turn on backup for this PC."
Click Next and save the revised plan.
-steve