I know about some Radeon and GeForce cards and there is one thing: Control panel. There is Windows Control Panel, and there is Gfx Card Control panel in Win Control Panel, open it (there is second way to open it: right-click in desktop - Ati Radeon Control Panel (If there is one)) and there is something about resolution. Scale: I think you have "w/o scale" option. Set it to "Scale with keeping ratio" or something like that.
If you have newer Catalyst drivers you have to access to the CP as vectar said, click on "Graphics", click on "desktops & displays", right click on your screen's icon at the bottom, click "configure" and chose "full screen" so you won't have anymore those black bands. If you want to turn back to normal click on "Preserve aspect ratio"