Steam games you wont have to reinstall. You can Install Steam over the old location on the HD and the Steamapps folder will still have the game files in it. You can go to your library in steam and opt to install again. It will check the folder and take a bit of time to verify, run any updates, and register the game with the OS. Saves you a lot of time instead of downloading. Origin games can do something similar, as well: you may need to target the game install folder though from the app. Things installed from disk or a stand alone installer may have to go through the full reinstall process though.