1. A method comprising: providing a computer-readable medium encoded with instructions that implement one or more customized execution environments, each customized execution environment providing an execution environment for a single application and exclusively managing a subset of hardware resources of a computer system, with no operating system abstractions or interfaces interposed between the customized execution environment and the subset of hardware resources; determining which hardware resources of the computer system are to remain under control of a resident operating system executing within the computer system and which of the hardware resources of the computer system constitute each subset of hardware resources of the computer system managed exclusively by one of the one or more customized execution environments; and partitioning the hardware resources among the resident operating system and the one or more customized execution environments by associating one or more partitions of the hardware resources with the one or more customized execution environments.