Updating to ios 6 and cannot complete the steps on phone - connecting to Itunes

The update to ios 6 is complete on my iphone 3gs, but now a screen appears on my phone indicating there's just a few more steps to follow. I press continue and the next step is to connect my iphone to itunes. I plug the iphone into our pc with a usb cord and I can see it coming up under the devices in our itunes account. But then it disappears. A few minutes later, it comes up again (as if it's updating or loading), to no avail...disappears again. I am unable to use my phone and would like to just finish the update and be on my way. PLEASE HELP!