We describe an architecture for reusing computable guidelines and the programs used to interpret them across varied legacy clinical systems. Developed for the PRODIGY 3 project, our architecture aims to support interactive, point of care use of guidelines in primary care. Legacy medical record systems in UK primary care are diverse, using different terminologies, different data models, and varying user-interface philosophies. However, our goal is to provide common guideline knowledge bases and system components, while achieving full integration with the host medical record system, and a user interface tailored to that system. In conjunction with system suppliers, we identified areas of standardization required to achieve this goal. Firstly, standardized interfaces were created for mediation with the legacy system medical record and for act management. Secondly, a standard interface was developed for communication with the User Interface for guideline interaction. Thirdly, a terminology mapping knowledge base and system component was provided. Lastly, we developed a numeric unit conversion knowledge base and system component. The standardization of this architecture was achieved by close collaboration with existing vendors of Primary Care computing systems in the UK The work has been verified by two suppliers successfully building and deploying systems with User Interfaces which mirror their normal look and feel, communicating fully with existing medical records, while using identical Guideline Interpreter components and knowledge bases. Encouragingly further experiments in other areas of clinical decision support have not required extension of our interfaces.