In this paper, we present THINK, our distributed systems architecture, and the research we have conducted to provide the system programmer with an architecture he can use to build efficient and secure operating systems. By specifying and implementing elementary tools that can be used by the system programmer to implement a chosen security policy, we prove that flexibility can be guaranteed in an operating system without compromising security. Our work focuses on protection against denial of service attacks which compromise the system fairness in resource multiplexing and can cause the system to stall due to resource starvation. We first briefly describing the THI NK architecture before positioning our contribution against related work. We then present the elementary tools we have specified to ensure quality of service in THI NK, before detailing the software memory isolation tool we have implemented and tested. We conclude by a concrete example of the utilisation of these tools.