See this article about backups and where they are stored on your computer: iTunes: About iOS backups
To save the backup, copy the backup folder to a save place or include it in your computer backup routine.
The folder where your backup data are stored varies depending on the computer's operating system. Make sure the backup folder is included in your periodic data-backup routine.
iTunes places the backup files in the following places:
- Mac: ~/Library/Application Support/MobileSync/Backup/
- Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
- Windows Vista and Windows 7: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
Note: If you do not see the AppData or Application Data folders, you may need to show hidden files (Windows XP, Windows Vista, Windows 7), or iTunes may not be installed in the default location. Show hidden files and then search the hard drive for the Backup directory.
Assuming you have Windows:
- Quit iTunes.
- Start an elevated command prompt (Start Menu>All Programs>Accessories>right-click "Command Prompt">choose "Run as Administrator")
- Once in the command prompt, type "netsh winsock reset" and hit ENTER.
- Reboot your machine.
- Restart iTunes.
- Try backing up your phone again.
If it still won't back up you probably have a corrupt backup on the computer and must delete it. First go to iTunes Preferences, Devices tab, select the backup and use the Delete button. Then try backing up your phone again.