Have you tried the 'Mr Alfa' solution. I think that should help you. That solution is for Windows 7. If you are working on some other operating system, then the following points will help you.
To resolve this quandary, delete the backup in iTunes Preferences and then create a new backup. If you would like to retain a copy of the backup, you can move the backup file to a different location. The backup is situated in the locations that are mentioned below.
On a Mac:
/Users/[username]/Library/Application Support/MobileSync/Backup/
On Windows XP:
\Documents and Settings\[username]\Application Data\Apple Computer\MobileSync\Backup\
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