1. A method of updating a computer system, comprising: comparing a list of files in a later master computer system image with a list of files in a prior master computer system image to construct a manifest list of files to be used in performing a differential system image update of a target computer system having the prior master computer system image installed thereon, wherein the later master computer system image represents a direct modification of the prior master computer system image, the manifest list of files defining modifications of the prior master computer system image necessary to arrive at the later master computer system image; associating file state information with at least a portion of said manifest list of files corresponding to both said prior master computer system image and said later master computer system image; and determining whether use of the manifest list of files to perform the differential system image update of the target computer system would be valid, wherein use of said manifest list of files is valid when said file state information associated with said portion of said manifest list of files concurs with file state information for corresponding files of said target computer system that is to receive said differential system update.