If you are using a HDTV (or actually even if not), this is most likely due to
Overscan.
Here is a great article that discusses all the in's and out's of HTPC Overscan compensation on ATI and nVidia video cards which should help. To further explain, here's a quote:
And for those of you unfamiliar with "overscan", it is simply the part of the picture that is cropped. Depending on whom you ask, others also describe it as the space that bleeds or "scans" beyond the edges of the visible area of the screen. Typical televisions can have a loss of up to 20% of the image due to cropping. This portion of lost image is what is commonly known as overscan. Technically speaking, the information of the "lost picture" is not actually lost, but it is outside the range of the visible area of your TV screen. ...
It is implemented deliberately on TV sets because of the different video input formats: composite, s-video, etc., all of which the TV needs for which to provide support. If overscan was not implemented as a factor of these different formats, there would likely be underscanning of different degrees on different TV sets.
But basically, you have to adjust the TV for use with a computer! You probably know how when you buy a TV, they advertize the screen size as two sizes (one official and one "viewable"). That's not because they exaggerate the screen size, its because the screen itself is actually the advertized size (in most cases), but the edges of it are covered up by the case plastic so that you don't see these "edge distortion" and picture size issues.