1. A method for operating a thread of a computer-executed process across boundaries of a plurality of virtualization contexts within an operating system, each virtualization context comprising an application execution environment and defining a context in which a computer-executed process executes which comprises its own virtual computing resources and which is isolated from virtual computing resources of other virtualization contexts, the each virtualization context intercepting and redirecting operating system calls from the executing process to provide an illusion that the executing process is executing within a native operating system environment of a computer, the method comprising: a thread operating in a first virtualization context and having access to first computing resources associated with the first virtualization context, the first computing resources being isolated from other computing resources of other virtualization contexts; the thread entering an entry point to code associated with a second virtualization context that has access to second computing resources associated with the second virtualization context; while the thread is operating on the code, detecting an event; and in response to detecting the event, associating the thread with the second virtualization context such that the thread has access to the second computing resources associated with the second virtualization context; wherein the associating with the second virtualization context is performed using at least a registered origin information.