Iphone 4 error 21 stuck in recovery mode

My phone Iphone 4 is having a few problems. It won't power up without being connected to the mains or connected via a computer. When it is connected it comes up a little symbol saying connect to Itunes, and Itunes says that it needs a system restore, When I try this it comes up error 21 and refuses to reinstall the system. Here is the things I have tried so far.

* Tried it on a difference pc
* Tried reinstalling itunes and I have the updated version
* I have tried renaming the file in drivers/etc didn't work
* I have tried holding it down and the screen goes black then the apple logo comes
back up and nothing happens.

Here is my 2 questions, is there anything else I haven't tried that can save my phone?
Is there anything I can do to fix this

And i have made an appointment with Apple Store to try and fix it, is there any chance that Apple could save my phone?

To the first question, i wouldn't try anything else b4 i went to the apple store and to the second yes there's a chance they could fix it, there not the smartest people the the world, but they usually knows a little about apple products