I'm going out on a limb and assuming that you are using Windows Live OneCare and that you are asking about starting a new backup set after having deleted the old backups from the destination drive. So, I've moved your post to the Backup topic folder.
If you aren't using OneCare and my assumption is all wrong, you're off topic for this forum.
Now, if you have deleted the old backups, you'll need to reset the backup plan settings in OneCare.
The following steps should do it for you:
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.
Go back to the backup tab and click Change Settings in the lower left once again.
Select your PC and click.
Click Turn on backup
Select the change location link and select the drive you want to use for backup
Modify the schedule as desired for "When"
Select the file types as desired for "What"
Click next and save the revised plan with the new selection.
-steve
