Accessing files on computers not running file sharing programs

Yesterday I was searching for documents. Some of the results were curious, so I downloaded some. (docs are small and quick to DL.) The files were patient information and medical records. Browse Host yielded the entire contents of her computer. Chat Host did not work. As it turned out, there was a good reason for that. Eventually I was able to talk to her; she is not running Limewire or any other file sharing program. She has no idea how her computer became open to the world. No one else uses her computer.
I spent most of the night learning that there are many (Since I found a few in 4 hours.) other computers out there giving the entire world access to their personal files. For example, I found a family in Washington State whose geneology, job applications, and other identity theft information is open to all. Stock trading information. Anything.
So all I can think of is virii and worms.
What gives? Anybody?