Assuming you have Windows, before deleting your backups do this:
- 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 again. If it still fails, then you probably have a corrupt backup that must deleted.To do this open iTunes and go to Preferences. On the Devices tab you will see a list of your backups. Click each one and choose Delete Backup. Then try backing up your phone again.
On your computer, go to iTunes > Preferences > Devices. Select the backup you want to delete and click the "Delete backup" button.
If you use iCloud backup, on your phone go to Settings > iCloud > Storage & Backup. Tap Manage Storage and tap your device's name. Then, you can:
- View the size of your backups and see the storage used by each app.
- Enable or disable Backup for each app.
- Delete Backups.
To learn what is and is not included in a backup, see http://support.apple.com/kb/HT1766