Interesting. Because both errors/conditions are mapped to ERROR_INVALID_HANDLE we need the creation time. I can work on a patch for that. Potentially I can also include OSX, Linux and BSD* implementations for methods involving os.kill(pid). That would be a broader task though. That also raises the question if there are other methods other than kill()/terminate()/send_signal() that we want to make "safe" from the reused PID scenario. 

> Also, unrelated but something I noticed. Using _active list in Windows shouldn't be necessary. Unlike Unix, a process in Windows doesn't have to be waited on by its parent to avoid a zombie. Keeping the handle open will actually create a zombie until the next _cleanup() call, which may be never if Popen() isn't called again.

Good catch. Looks like it deserves a ticket.