When you double click my computer and double click the Local Disk C: you will see a folder named Program Files all the program and data are store in this folder if you choose to install the program in Disc C. A lot of people will store important data in other disk such D or E if they have two disk. Normally
Operation System (OS) is install in Disk C and if it was corrupt or need to be
reinstall the OS all data stored in this disk would be erase during formatting.