1. A method comprising steps of: A) preparing on a preparation machine an installed image comprising multiple semantic levels of an application and its non-operating system dependencies; B) virtualizing the installed image on an execution machine to produce a virtually installed image by using a hierarchy of selective virtualizers, wherein the virtualizing is selective such that not all operations of the executing software at any particular level are virtualized; wherein each of the virtualizers is responsible for partially virtualizing different semantic levels of the application; wherein virtualization occurs where needed and more than one virtualizer can be active in a process; 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; C) storing the shards in an associative backing store; D) 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; E) 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; and F) integrating the execution of the software assets with the associative backing store, such that software information about said software assets and their versions can be collected.