Help please Iphone 3g stuck on apple logo afer resetting

So I just purchased a 3g from somebody. When I tried to sync it to my Itunes it told me This iPhone cannot be used because required software is not installed. So I decided to reset the settings since this guy did not remove his contacts and other stuff. It took about an hour and now it is stuck on the apple logo. I have no idea what to do. I have tried on holding the power and home buttons and connecting the USB cord and the iTunes + Arrow + cable screen appears and a message pops up on Itunes prompting to install a software but after I continue it does not recognize the phone. Please help I dont know much about Iphones.