You cannot point the OneCare backup to an internal hard drive within the PC without a workaround involving Centralized backup and creating a Network Share on the drive. I don't recommend it. The concept of a backup is to make a safe copy of your data on other media.
You can backup to CD/DVD, an External Hard drive, some flash based media, or a Network Share.
If you do not have any of these available and are willing to accept the risk of data loss in the event of a hardware failure, you can disable backup.
Open OneCare
Click Change Settings
backup tab
Click Configure Backup
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
