She attempted to port over files from an XP to the Vista without a partition and the files were overwritten.
Can you clarify what you mean? Are you saying the hard drive on the XP machine had multiple partitions but the Vista machine does not? As Allan said, "Partitions have nothing to do with overwriting files." But, if you want to add partitions to the Vista machine, that can be done and would be a good idea, especially where important business data is involved.
What type of files were/are you supposed to transfer from one computer to the other? Excel, Word, an accounting application, etc.?