If you handle legacy data, keep a copy of this installer archived. If you are a new CAD user, study V5R20 to understand the foundational logic upon which all modern Dassault products are built. Just remember:
If you have legitimate access to this software (via a Dassault Systèmes license or legacy archive), here is the standard installation workflow: dss catia p2 v5r20 ga sp0 win32 win64 multi