i have ipod touch 2nd generation with IOS 2.2.1 and not able to upgrade.

I have updated windows 7 and have the latest itunes installed. when I connect my Ipod touch. the Itunes gives me an option to upgrade. When I upgrade, it downloads the software, extracts and then finally it gives the error, "The iPod "xxxx" could not be restored. An unknown error occured (-1)".