ITunes could not restore iPhone. Unknown Error 21?

I've tried everything, and it won't work. It won't restore my iPhone 3. ITunes will not restore it. Error 21. I've uninstalled then reinstalled iTunes. Also, how do I claim ownership of host file or whatever? I tried watching this video but it's such bad quality I couldn't do it. I'm just so frustrated, please help me.

You may need to uninstall and reinstall components of itunes, once you remove all components of itunes on a Windows Comp you can install each component separately in the order online in HT1923. On a mac you need to download the latest version and overlap original.

Download and save a copy of the iTunesSetup.exe (or iTunes64setup.exe) installer file to your hard drive:

http://www.apple.com/.../download/

Download and install the free trial version of WinRAR:

http://www.rarlab.com/

Right-click the iTunesSetup.exe (or iTunes64setup.exe), and select "Extract to iTunesSetup" (or "Extract to iTunes64Setup"). WinRAR will expand the contents of the file into a folder called "iTunesSetup" (or "iTunes64Setup").

Go into the folder and doubleclick the AppleMobileDeviceSupport.msi (or AppleMobileDeviceSupport64.msi) to do a standalone AMDS install.

(If it offers you the choice to remove or repair, choose "Remove", and if the uninstall goes through successfully, see if you can reinstall by doubleclicking the AppleMobileDeviceSupport.msi again.)

Does it install (or uninstall and then reinstall) properly for you? If so, can you get a normal iTunes install to go through properly now?

If instead you get an error message during the install (or uninstall), let us know what it says. (Precise text, please.)