Very large size Peer-to-Peer systems are often required to implement efficient and scalable services, but usually they can be built only by assembling resources contributed by many independent users. Among the guarantees that must be provided to convince these users to join the P2P system, particularly important is the ability of ensuring that P2P applications and services run on their nodes will not unacceptably degrade the performance of their own applications because of an excessive resource consumption. In this paper we present Interceptor, a middleware-level application segregation and scheduling system that is able to strictly enforce quantitative limitations on node resource usage and, at same time, to make P2P applications achieve satisfactory performance even in face of these limitations.