How can I apply environment variables without reboot?

Is there a way I can apply changes to environment variables without rebooting on Windows 7?

If you set an environment variable with the SET command in a command interpreter, it applies right there and then, to that process' environment. No rebooting, no logging off and on — nothing more required.

The environment variables that are held in the registry are not, in fact, environment variables at all. They are a template. Coöperating programs read from the template and create/modify their own actual environments from it. They know when the template changes because there's a windows message that is broadcast to all top-level windows to say that this has happened. You need to broadcast that message. All coöperating programs, including Windows Explorer, will re-read the template and adjust their environments, that will be inherited by any new processes that they spawn from that point onwards, accordingly.