I'm not sure if that's an input signal problem or a display problem...
Are you using the standalone SlingPlayer app ? If so (and if it's a display problem) try poking around the View menu, look for a Fit TV to Window toggle along with the other settings in there.
Can't speak to Windows, but on Mac OS X second monitors that the OS picks up as video displays usually have an Overscan toggle in the Monitors control panel. If you have something similar, maybe shut that off ?
If you have any other observations or details, I'd like to hear them. You have an interesting question.