Starting with Vista and Windows 7, the type of window NetHack.exe uses can
no longer be expanded to fill the entire screen.  Here is what you can do
about that:

You can improve things somewhat by picking a bigger font so that the window
gets bigger to hold the same number of lines and columns.  Click on the upper
left corner of the window border and pick "properties" from the drop down
menu.   [In Windows XP and earlier, you could pick "maximize" from that menu
instead and get the result you want; that doesn't work now.]

Then on the properties widget, pick the Font tab.  When I do that on Vista, I
see the choice of two fonts:  Lucida Console and Raster Fonts.  For Raster
Fonts, there's a choice of sizes 6x8, 8x12 and so forth.  None is particularly
satisfactory, although one or two of the biggest ones are better than the
default.  But Lucida Console gives better results.  The sizes there are in
points or some such unit rather than pixel XxY grid amounts, and the best one
to pick will depend upon your monitor size and the screen resolution it's set
for.  In addition to maybe getting a better fit for you, Lucida Console is
easier to read than raster font, at least for me.

Once you find the best size, you can create a shortcut and apply the property
changes to that and then save it.   Then run nethack via the shortcut rather
than directly starting nethack.exe.  That way you won't have to reset the font
every time you run nethack.