Heya! ^_^ I can gladly help with this.
A couple of things to note:
A) Game companies can save their files in many different places, so depending on what game is being backed up, they will be in a different place. I'll provide a generic "Rough assumption" of where they are, unless you have a specific game in mind.
B) Game saves that save screenshots can tend to get rather large, so make sure to have a fairly big USB stick with lots of empty space!
Now, because you mentioned some choice Nitro+CHiRAL selections I'll help with those first.
Togainu No Chi: This one is kinda easy, because the game saves it's files in the same folder the game was installed. So to get there you find the folder you installed the game to and then copy the folder named "
SAVE".
(Here are steps to find the folder, if you don't know where it is. Behind spoiler tag so the post doesn't get too big...)
And now for DMMd and it's fanbox DMMdrc: Most newer Nitro+CHiRAL games save their files in the same folder so it'll make backing them up easier. It doesn't matter where the games were installed, as the save files are in the same place on every computer.
First click on the Start button and in the search box type this:
%APPDATA% then press enter. The window will magically open up the save files for nearly all your computer programs! Here you will find a good portion of the save files for your games, as long as you know the name of the company who made them, or the game name.
For now, just start typing out the word 'nitro' and the window will scroll to the folder named "
NitroplusCHiRAL". This is the folder you need to copy to your USB drive.
Once those are backed up, you do everything in reverse on your freshly formatted computer. Just make sure to install the games FIRST, before restoring the save files.
If you have any other specific games, let me know and I can tell you where they might be. But a good portion of them save to the same folder as it was installed, or that AppData folder.
Good luck!