1. An information processing system comprising: a preparation machine comprising an installed image; an execution machine on which the installed image is virtually installed; and a virtualizer virtualizing the installed image on the execution machine, producing a virtually installed image by using a hierarchy of selective virtualizers; wherein the virtualizing is selective such that not all operations of executing software of the installed image at any particular level are virtualized; wherein the virtualizing comprises steps of: defining virtualization of the virtually installed image by characterizing a hierarchy and selectivity of the virtualization, such that said virtualization is both selective and hierarchical; emulating on the execution machine a platform native to the preparation machine; emulating on the execution machine a virtual machine of the application; emulating on the execution machine other applications interacting with the application; dividing software assets of the installed image into shards, said shards comprising variable size, semantically-determined pages; executing the software assets on the execution machine as if said software assets were installed on said execution machine, wherein the executing step comprises: intercepting requests at multiple levels; and redirecting the requests to code that emulates the behavior of the application installed on the preparation machine; and integrating the execution of the software assets with the virtualizers, such that execution information can be collected efficiently about execution problems on the execution machine.