Grids are emerging as a global computation infrastructure that enables the sharing, exchange, discovery, selection and aggregation of geographically distributed, heterogeneous resources. In this paper, we studied and implemented a simple proactive service broking middleware (PSRB) for building PC-based Grid that promises a cost-effective approach for high performance computing. It employs a robust mediation server group to centrally serve Grid nodes and clients by exchanging their service requests and responses as a message broker. At the same time, it provides a few resource agents to proactively monitor the resources on Grid nodes and forecast their future availability. Predictions are accepted as the primary fact for PSRB to select the best Grid node for job assignment. Based on all these, PSRB provides a fairly reliable infrastructure to build PC-based Grid that allows the heterogeneity and autonomy of Grid nodes and ensures impartial resource utilizations.