This is easy if you properly prepared yourself. Since the early 1990s, I have always partitioned my HDs for performance and organizational reasons. If ya had the OC on 1 HD partition and ganes on another it's a simply copy paste thing. Never let stuff go in C:\ProgramFiles.
If I understood you correctly, you installed Windows on the new SSD. Windows and all ya games are still installed on the HD. Easy (short term way) would be to simply install the games over themselves but would leave a mess. I'd suggest:
1. Create a new folder called Games .... I assume the HD is now D:\ So this would be D:\Games
2. Pick ya first game and find it's install folder, say:
D:\ProgramFiles\BigCheesegameCompany\BananaJuggler 3
3. Use CUT / PASTE to cut the [BigCheesegameCompany\BananaJuggler 3] folder from
D:\ProgramFiles\BigCheesegameCompany\BananaJuggler 3
to
D:\Games\BigCheesegameCompany\BananaJuggler 3
4. "Rinse and Repeat" for each game folder.
5. Now saved games are oft installed in a user directory .... will now be D:\Users since HD is D:\ . You will have to check and see where each game stores these and other files if you want to retain. What was in your user file on the HD will have to go in the same user file on the SSD.
6. Go thru the rest of your HD and save what you want to keep and delete what you don't. Of course best to make a backup before starting any of this.
7. Defrag the drive
8. Install all the games to the locations into the same folder.... when you start the install and it wants to default to
C:\ProgramFiles\BigCheesegameCompany\BananaJuggler 3
change it to
D:\Games\BigCheesegameCompany\BananaJuggler 3
The install will set up any appropriate registry entries if needed and will not overwrite any newer files which contain settings or anything else that you customized.