1. A method of updating a computer system, comprising: a) comparing a list of files in a new computer system image with a list of files in a former computer system image to construct a manifest list of files comprising a subset of said new computer system image files and said former computer system image files; and b) associating information with said manifest list to facilitate determining whether a differential update to a computer system is valid, wherein said differential update comprises differences between said new computer system image and said former computer system image.