Iphone problem! Unknown error 21?

My iphone 4 just randomly turned itself of and i tried to restore it but it said 'unknown error 21' can someone please tell me how to fix it?

Are you using a Windows computer? If so, this error usually means your security software is interfering with the restore process. Disable the security software temporarily and restart the restore. If you get any other errors, take a look at this: http://support.apple.com/.../kb/TS3694

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