General purpose Operating Systems do not provide effective mechanisms for application processing reservation. To overcome this limitation, some initiatives aim at guaranteeing processing by instrumenting kernels or by isolating the performance through the creation of virtual machines. However, these proceedings may be impractical if the kernel code is not available or high overheads should be avoided. As will be described in this paper, CPUReserve works differently from these approaches. It is a processing reservation system that runs at user level. CPUReserve allows the configuration of the active scheduling policy and simplifies the development of new ones. Thus, it can be used in many computing scenarios such as distributed and shared environments.