In this paper we study the use of idle cycles in a network of desktop workstations under unfavourable conditions: we aim to use idle cycles to improve the responsiveness of interactive applications through parallelism. Unlike much prior work in the area, our focus is on response time, not throughput, and short jobs - of the order of a few seconds. We therefore assume a high level of primary activity by the desktop workstations’ users, and aim to keep interference with their work within reasonable limits. We present a fault-tolerant, low-administration service for identifying idle machines, which can usually assign a group of processors to a task in less than 200ms. Unusually, the system has no job queue: each job is started immediately with the resources which are predicted to be available. Using trace-driven simulation we study allocation policy for a stream of parallel jobs. Results show that even under heavy load it is possible to accommodate multiple concurrent guest jobs and obtain good speedup with very small disruption of host applications.