Change the iPhone/iPod backup to another drive in windows OS

1. Move the "Backup" folder from the "C:\Users\\AppData\Roaming\Apple Computer\MobileSync\" location and place to an external drive, let's say to "D:\itunes\".

2. Open command prompt from Start --> Run and type -
mklink /J "C:\Users\\AppData\Roaming\Apple Computer\MobileSync\Backup" "D:\itunes\Backup"

after successfully done, u will get a msg -
Junction created for C:\Users\\AppData\Roaming\Apple Computer\Mobile
Sync\Backup <<===>> D:\itunes\Backup"
and will create a shortcut folder "Backup" under "C:\Users\\AppData\Roaming\Apple Computer\MobileSync\" 
Have fun!!! now you can have GBs of backup in external drive with out affecting to the OS installed drive.