A system for opening a window in a casing string (42) positioned in a wellbore (32). The system includes a hydraulic pressure intensifier (116) having a housing and a piston assembly with a differential piston area. The piston assembly is longitudinally shiftable relative to the housing and is initially secured relative thereto to prevent longitudinal movement therebetween. An anchor assembly (114) is operable to longitudinally secure the hydraulic pressure intensifier (116) within the casing string (42). A window opening tool (118) operably associated with the hydraulic pressure intensifier (116) is operably engagable with the casing string (42) such that when the anchor assembly (114) is longitudinally secured within the casing string (42) and the piston assembly is unsecured relative to the housing under hydrostatic pressure, longitudinal movement of the piston assembly transmits a force to the window opening tool (118), thereby opening the window in the casing string (42).