Shoutbox

It does look a lot like one of those password stealers as msg_plus_freak suggested. I've seen a lot of them before and many of them feature differant ways of tricking the victim into entering their email and password for them to retreive, such as faking a messenger update which prompts you to sign in with your email and password.

If anyone has sent you any files lately that didn't seem to do anything upon trying to open them, this could be the server file which is allowing them to connect to your computer and cause the popup to be displayed. Look for 'server.exe' on your processes list or any other suspicious looking files, also look in your startup list. And remove anything that seems suspicious. (only if you're sure it's safe to do so)