See Here > http://support.apple.com/kb/HT1808
You may need to try this More than Once...
Be sure to Follow ALL the Steps...
Some users have reported that as many as 8 or 9 attempts were necessary before success.
Pay particular attention to Steps 3 and 4.
Restore loop (being prompted to restore again after a restore successfully completes)
Troubleshoot your USB connection. If the issue persists, out-of-date or incorrectly configured third-party security software may be causing this issue. Please follow Troubleshooting security software issues. .
Next try placing in DFU mode and then restoring.
Then try restoring on another computer.
Last, make an appointment at the Genius Bar of an Apple store.