A management service can install one or more third-party agents and any number of customizations for an operating system/virtual machine at the same as the system/machine is installed. In one implementation, a management service places (e.g., from a remote location) an operating system/virtual machine image in a mounting location. Before installing the corresponding system (OS or virtual machine), the management service also inserts one or more third-party agents into the image, as well as any number or type of policies and customizations. The management service then directs installation of the combined image (including rules, policies, and customizations) so that the operating system, third-party agents, and provided customizations, are all installed together during the same installation process. Accordingly, the management service allows for agents to reliably and immediately begin executing with installation of the operating system/virtual machine.