Global Computing achieves high throughput computing by harvesting a very large number of unused computing resources connected to the Internet. This parallel computing model targets a parallel architecture defined by a very high number of nodes, poor communication performance and continuously varying resources. The unprecedented scale of the Global Computing architecture paradigm requires to revisit many basic issues related to parallel architecture: programming models, performance models, and class of applications or algorithms suitable to this architecture. XtremWeb is an experimental Global Computing platform dedicated to provide a tool for such studies. This paper presents the design of XtremWeb. Two essential features of this design are multi-applications and high-performance. Accepting multiple applications allows institutions or enterprises to setup their own Global Computing applications or experiments. High-performance is ensured by scalability, fault tolerance, efficient scheduling and a large base of volunteer PCs. We also present an implementation of the first global application running on XtremWeb.