Error installing IOS 5.1

I've just got an e-mail with a question. It is probably not an unique problem, Apple's Servers Giving Errors to iOS 5.1 Upgraders.

Error 1600 - error in restoring

Error 1600 or 1603, 1604 can appear when trying to restore to iOS 4 custom firmware in iTunes. To fix this problem, try the following steps. Make sure that iPhone is disconnected from your computer and that iTunes is closed. Open folder C:\Documents and Settings\yourname\Application Data\Apple Computer\iTunes\iPhone Software Updates. The files from this folder need to moved somewhere else Launch iTunes and put your iPhone in recovery mode Proceed with restore and iTunes will download the update online.