There are a few possibilities:
Draw a textbox, type each letter followed by a shift+enter.
Or draw a textbox, type your word(s) as you would usually do. Format the
text so that it is center aligned (Format - textbox - textbox tab), drag
with the lower right handle of the box so that it is higher and smaller.
Third option, use WordArt, there are a few possibilities to create vertical
text on your far left.
Luc Sanders (MVP - Powerpoint)