First off do a search on your PC (F3) for the file that's already installed that has the same name as the one you downloaded. When you find it, rename it and put the new one in its place.
If it still doesn't work the best thing to do is just to reinstall the game - don't worry, it won't affect your saves.
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"Pointing and clicking becomes tiresome" - Gamespot, on Strong Bad's point and click game
Linear Mode
