I said use your nvidia controle panel (your graphic drivers), unless your ATI which I can not help you if you are. With advanced options enabled you can edit digital screen scaling (which you have I think as analogue ones do not suffer from not filling the screen usually) and enable it. It is called something like Desktop scaling and can easilly cause that problem with games like Diablo II and maybe
WC3 if left off (do not scale).
However run "regedit" to open up the registry editor. Then manually find the
WC3 options (searching can speed this up) and then edit the fields for screen resolution. Remember to make sure you set them to display as decimal and not hex and then enter your screen's resolution into the 2 fields (1 dimension each). Apply the changes (pressing ok) and then run up
WC3 and it should be now at the resolution of your screen and full screen.
Worked for me and now I have
WC3 at 1680*1050 on my vista machine.