Virtualization, a technology first developed for partitioning the resources of mainframe computers, has seen a resurgence in popularity in the realm of commodity workstation computers. This paper introduces Maestro- VC, a system which explores a novel use of VMs as the building blocks of entire Virtual Clusters (VCs). Virtualization of entire clusters is beneficial because existing parallel code can run without modification in the virtual environment. At the same time, inserting a layer of software between a virtual cluster and native hardware allows for security enforcement and flexible resource management in a manner transparent to running parallel code. In this paper we describe the design and implementation of Maestro-VC, and give the results of some preliminary performance experiments.