Can't update to iOS7.1

Hello,

I've an iPhone 4S with iOS 7.0.6 (11B651), my wi-fi antena does not work so I'm stuck with iTunes since I can't update via 3G... Problem with iTunes is I'm getting stuck forever at "Preparing iPhone for update". iTunes extracts the firmware, backup my data and then infinite wait...

I've reinstalled iTunes, I've downloaded several times the .ipsw (even from the dev. page), I've tried DFU mode, different USB cables and ports but nothing seems to work.

I'm using Windows7 and this is the first time it behaves like this, how can I solve this?

Thanks for your time.