in addition to setting the correct resolution, you will also need to ensure that your TV is not "overscanning". Basically, your computer might be outputting the correct pixel count (maybe 1366x768?) but the TV is displaying something else (like 1284x720 for eg). This reduced resolution is not the native resolution of the screen and would cause the problem you describe.
This is a common phenomenon and it's designed to trim the edges from a TV signal that might not have displayable content at those extremities. However, you don't want this enabled for your signal from the PC. As long as you've got the right resolution selected in Windows, the problem lies with your TV's settings.
I don't know if this is what's happening in your situation but usually there's a menu option on your TV to disable overscanning. The exact instructions vary from model to model, though. What model do you have?
This problem was prevalent when LCD TVs first appeared and in some cases couldn't be corrected. Newer TVs should be ok, though.--