1. A system for securely transmitting data between a roaming computer and a managed network service over a shared public network, the system comprising: a roaming computer; a server computer connected to the roaming computer via the public network, the managed network service being accessible from the server computer; a client agent installed on the roaming computer for creating a secure connection with the managed service and for transmitting data from the roaming computer to the managed service via the secure connection; a connection component of the managed network service installed on the server computer for cooperating with the client agent to create said secure connection; the client agent and the connection components of the managed service being operable, on an initial request from the roaming computer to the managed service to negotiate the secure connection using certificate based authentication.