error code 28

Try uninstalling the security software on your computer and then testing again. Then reinstall it afterwards.

iTunes: Specific update-and-restore error messages and advanced troubleshooting

Error 20, 21, 23, 26, 28, 29, 34, 37

These errors typically occur when security software interferes with the restore and update process. Follow Troubleshooting security software issues to resolve this issue. In rare cases, these errors may be a hardware issue. If the errors persist on another computer, the device may need service.

error code 28 http://support.apple.com/kb/TS3694#error28

See this support document http://support.apple.com/kb/ts1275. Generally that error has to do with security software issues. You may need to temporarily disable your security software.