http://superuser.com/questions/18039...ion-on-windows
But basically it amounts to this:
1. Ensure iTunes is closed, and move the backup directory from:
C:\Users\{UserName}\AppData\Roaming\Apple Computer\MobileSync\Backup
to wherever you want. Ensure that there is no more directory called Backup in MobileSync.
2. Open a command prompt (Start, type cmd in the search box), and navigate to the directory:
C:\Users\{UserName}\AppData\Roaming\Apple Computer\MobileSync
Run the following command to create a link to the new backup location:
mklink /J Backup D:\Whatever\Backup
Where D:\Whatever\Backup is the drive and directory where you copied the Backup folder to in step 1.
That should be it.