I'll purposefully be "naive"...
I think my previous answer was clear enough for a one-way 99% working solution, but I'll put it down in a more explained and accurate way.
"Tl;dr" at the bottom, if you want the short version.
- If you're trying to reinstall retail games, you're better off deleting anything left behind from your previous OS. By retail, I mean all those games you may own which do not require Steam/Origin/Uplay.
You may try keeping savegames and try copying those once the game has been ran at least once, but it doesn't always work...
- If you're transferring a Steam game, you can keep them. Once you've downloaded and reinstalled Steam and once you defined the default installation directory for it, close it down. Copy...