Hi manong30 and welcome to the forums.
GTA 4 saved game is located in:
C:\Users\username\AppData\Local\Rockstar Games\GTA IV\savegames\user_#
If you are going to back up your game just copy all the "user_#
After installing the OS and the game:
- Sing-in to your account and start a new game.
- Skip the intro of the first mission and then exit the game.
- Go to the path above and it should be 2 folders the "user_invalid" and "user_#"
- Open the "user_#" folder and paste your "SGTA4#" file.
Now launch the game and it should load your previous saved game.
If you don't see the AppData folder in your computer please take a look at this
Let me know if you got any mo' question.