When trying to restore the iPhone in iTunes for Windows, you get an error:
Unable to restore phone. Unknown error (6)
To resolve this problem, try the following steps:
1. Open the following location:
C:\Documents and Settings\username\Application Data\Apple Computer\iTunes\iPhone Software Updates
Windows 7 / Vista:
C:\Users\username\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates
2. Delete files that end with a .ipsw.
Now hopefully you can restore your iPhone without getting the Error 6 message.