Global Computing achieves highly distributed computations by harvesting a very large number of unused computing resources connected to the Internet. Although the basic techniques for Global Computing are well understood, several issues remain unadressed, such as the ability to run a large variety of applications, economical models for resource management, performance models accounting for WAN and machine components, and finally new parallel algorithms based on true massive parallelism, with very limited, if any, communication capability. The main purpose of XtremWeb is to build a platform to explore the potential of Global Computing. This paper presents the design decisions of the first implementation of XtremWeb. We also present some early performance measurement, mostly to highlight that even some basic performance features are not well understood yet.