Word: Left and right align on one line

Select Tabs from the Format menu and click the Clear All button. In the Tab stop position box at top, enter the width (in inches) of the editable area of your document. You can get that value by looking at the right-hand end of the ruler above the document. Check the Right option and click OK. Now type some text, press the Tab key, and type some more text. The text before the tab is left-aligned; the text after the tab is right-aligned. Easy! If you want to end the right/left alignment after you've typed some lines, open the Tabs dialog again and click Clear All.

The drill is almost the same when you apply this effect to existing text. First, highlight the lines you're going to change. Use Find/Replace to replace any tabs (represented by ^t) with spaces. Then proceed to set the right-side tab as above. Finally, insert a tab character at the desired location in each line.
There are some variations on this technique that may prove useful. When you create the right-aligned tab, you can choose from one of several Leader styles. Word will fill the space before the right-aligned portion with dashes, dots, or underscores. You can also set a tab in the middle of the ruler and define it as a Center tab. Now enter a line of text split by two tabs. The first part will be left-aligned, the middle part will be center-aligned, and the last will be right-aligned.