You can install the Steam client and point it to look for games on your HDD. It will see the games, though a lot of them are likely to be labelled with yellow text saying update required, just run the update and it will validate your files and won't download anything unless there is an update for that particular game. Keep in mind you will have to run first time setup for all your games, though you shouldn't have to redownload anything. Some game may decide that they need to redownload though, Steam can be a bit weird that way, but most of them will be usable again, just run the game and let it do the first time setup again.