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:

Windows XP:

C:\Documents and Settings\username\Application Data\Apple Computer\iTunes\iPhone Software Updates

Windows 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.