2003

Error 2000-2009 (2001, 2002, 2005, 2006, 2009, and so on)

If you experience this issue on a Mac, disconnect third-party devices, hubs, spare cables, displays, reset the SMC, and then try to restore. If you are using a Windows computer, remove all USB devices and spare cables other than your keyboard, mouse, and the device, restart the computer, and try to restore. If that does not resolve the issue, try the USB issue-resolution steps and articles listed for Error 1604 above. If the issue persists, it may be related to conflicting security software.

Sometimes internet firewalls are stopping itunes to connect to apple servers.So, make sure to disable your firewall and if the problem remains try this http://support.apple.com/kb/HT1808