This is called 'overscan'.

You will notice that on computer monitors that there is usually a small black border aroudn the edge of the screen. When watching TV, there is no black border, as it isn't considered acceptable on a television.

To ensure this does not occur, televisions never show the entire picture. There is always a tiny tiny part of the picture that is off the edge. You have never noticed it until you hooked up a computer.

You will need to reduce the overscan. Luckily, you have an ATI graphics card. In the ATI control panel there usually is an HDTV tab (or section) that will let you reduce you resolution in non standard increments.

For instance, instead of running 1280x720, it is adjusted to 1268x712 (or something like that, I'm not in front of that PC right now).

This link explains overscan. Ignore the pricey hardware solutions as you shouldn't need it with the ATI card.
http://www.highdefinitionblog.com/?page_id=127

EDIT: saw you fixed the problem. I'll leave the post here for others.