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 your backup content in that folder and log back on. Steam should resynchronize the library itself.
- I don't know if Uplay and Origin use the same system that Steam puts up, so try the same thing and otherwise delete them.
- For Steam/Origin/Uplay games, redownloading the game from scratch will bring you no additional hassle. For all their games that you, however, transfer from a PC to another, it's your job to eventually update DirectX, Visual C++ redistributables and any extra requirement the game may have.
Tl;dr: delete and redownload every single game from Steam/Origin/Uplay or install from their discs, if they're retail copies.
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