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For more information on this, take a look at:
iOS: Troubleshooting backup issues in iTunes
"iTunes could not back up iPhone because the backup could not be saved on the computer"
perform the rest of the steps in this article
Best of luck,
Delete all the exisiting backups by navigating to:
C Documents and Settings\USERNAME\Application Data\Apple Computer\MobileSync\Backup
make sure show hidden files and folders is enabled: http://www.bleepingcomputer.com/tutorials/how-to-see-hidden-files-in-windows/
delete all the backups and then re open itunes and run backup again.