I know why your sister and Natty_sem having this problem. The reason is pretty straightforward: we can only create four primary partitions in any Windows operating system or three partitions and one extended partition. As you mentioned that your sister receives two different messages, but both messages are the main cause for this problem. They both stop her to create new partition. Those error messages indicate that the hard disk already contains four primary partitions. But don't worry, there is an effective solution for this situation. As Windows 7 doesn't allow your create new partition without changing basic disk from dynamic until you delete one partition, so this must not be useful. You need to take help from Windows 7 partition manager. This will help you create extra partition without any limitation.
Hope this will be helpful.