computationally intensive task makes gui not updatable

Hi all, I have a windows app that performs a computationally intensive task (encoding a lot of files). When I press the button to start the task, it seems that the app somehow hangs, in the sense that it doesn't update the gui, and doesn't repaint if another windows app covers and uncovers it. This is happenning until the taks is finished. To correct this, do I have to use threads? thnx, geo