On the hub PC, open OneCare, click on Change Settings, Backup and configure centralized backup for that PC by placing a check mark in the box for "use centralized backup."
You may need to reset the backup plan on that PC first.
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.
After you've saved the plan on that PC, go to the hub PC and follow the steps above to create a plan for that PC and*do* select Centralized Backup - assuming that you want to use backup on this manner and are planning on simply backing up all PCs to a Network Share, but not use Centralized Backup in OneCare.
-steve