Hmmmm,

Strange indeed. My gut feel is that it isn't malware, as that tends to be covert rather than blatantly advertising itself? It might be a prank or joke, or even an MSN update that hasn't worked properly?

You might try updating your AV, going into safe mode and scanning your MSN folder?

Also try right clicking on the white box and see if you get an option to display properties or the page source (sorry I don't do MSN) there might be a clue there.