We need you to be more specific. This way, other forum users will be able to help you with accurate information. We need to know the operating system of both computers and also if you are using Watch at slingbox.com
In the meantime, I suggest to check the aspect ratio that you are using with the Player, have a look at this,
Hi Michael - I have the exact same problem (on an iMac). It worked perfectly before changing my cable box but now has the "black border" around the picture, both on the Slingbox screen and on my computer screen when going to full screen mode. Were you able to figure out how to correct the problem?