Because you use DirectInput I assume that you are using DX7. You have two different choices. When you use buffered input just clear the buffer when you want the user to input something. Then when he finishes his input (button click or enter ,etc.) just read the buffer and store it into a string. When you use this way the user will not be able to see what he inputs. Or you use the non-buffered way. After each frame you look what keys are pressed insert them into a temporary buffer. This buffer you draw to the screen by either using your own text engine or DrawDebugText(very slow).
Hope this helped,
Yoshi