Regretfully there's nothing you can do through Selenium here. This window is from the so-called basic authentication, where the user is expected to enter a username and a password and is handled at http level.
The control you see - the dialog box - is an OS component, part of the browser executables itself; as such, Selenium cannot interact with it - it is sandboxed from it (on purpose, for security).
There are couple of workarounds; credentials for basic authentication can be passed through the url - if the target system does not prohibit that explicitly. That's done through a special format of the url:
http://username:password@example.com/
Another one is to use a library for desktop automation - AutoIt, or Java's Robot. But this has 2 (huge for me) drawbacks - the solution is going to be OS dependant (e.g. one for Windows, and different for Mac, Linux), and mainly - you cannot employ it in remote selenium session - you have to have that desktop automation running in the remote machine, with a desktop session it intercepts, and accessible from your execution machine.