The huge amount of resources available in the Grids, and the necessity to have the most updated experiment software deployed in all the sites within a few hours, have spotted the need for automatic installation systems for the LHC experiments. In this paper we describe the ATLAS system for the experiment software installation in LCG/EGEE, based on the Lightweight Job Submission Framework for Installation (LJSFi). This system is able to automatically discover, check, install, test and tag the full set of resources made available in LCG/EGEE to the ATLAS Virtual Organization in a few hours, depending on the site availability. The installations or removals may be centrally triggered as well as requested by the end-users for each site. A fallback solution to the manual operations is also available, in case of problems. The installation data, status and job history are centrally kept in the installation DB and browseable via a web interface. The installation tasks are performed by one or more automatic agents. The ATLAS installation team is automatically notified in case of problems, in order to proceed with the manual operations. Each user may browse or request an installation activity in a site, directly by accessing the web pages, being identified by his personal certificate. This system has been successfully used by ATLAS since 2003 to deploy about 60 different software releases and has performed more than 75000 installation jobs so far. The LJSFi framework is currently being extended to the other ATLAS Grids (NorduGrid and OSG).