Moving the iOS device backup location
Open a command prompt by hitting the start button and typing CMD<Enter> in the search box that opens up, or with Start > Run on older Windows.
To move the current backup folder from C: to D: (for example) type in this command and press <Enter>
Move "C:\Users\<User>\AppData\Local\Apple Computer\MobileSync\Backup" "D:\Backup"
Where <User> is your Windows user name.
To make iTunes look for the data in the new location type in this command and press <Enter>
MkLink /J "C:\Users\<User>\AppData\Local\Apple Computer\MobileSync\Backup" "D:\Backup"
If your preferred drive has a different letter or you already have a folder called "Backup" then edit "D:\Backup" accordingly in both commands.
If you have Windows XP then you'll need a third-party tool such as Junction to link the two locations together instead of the MkLink command. The source folder is C:\Documents and Settings\<User>\Application Data\Apple Computer\MobileSync\Backup