In Word, if you want to adjust letter spacing, go into the Font settings for some selected text (such as the line that is giving you problems), and choose Character Spacing under Advanced. Then select Expanded or Condensed and input as small a number as you can to get the results you want. If you expand, it will fill the empty space better, and if you contract, it might allow another word to flow onto the line and make it look better.
To the naked eye in print, small adjustments are hardly noticeable, yet they can significantly improve the look of justified text. If you make large adjustments, it can look weird, so it's best to be conservative. You can adjust in 0.1 pt increments, the last time I checked.