I always used ntbackup on WindowsXP for saving my data on a daily basis.
My backup type is incremental.
Now my external hard disk storage is full.
How can I delete old backups?
Delete the bkf files like you would any other file. Take note that deleting the bkf files may limit your ability to restore specific files\folders due to the backups being incremental backups rather than full or differential backups.