Products in CRM could be materials such as PC, Warranty, Financing and Service. The installed base is a representation of products and related services in a particular location. For eg. a building would have servers, printers, fax machines and software installed in different floors which becomes your installed base.
Simply put it this way, Products are used by sales for generating revenues for the company and Installed base is used by service to service products and to check contracts and agreements related to those products.
Hope that helps.