ITunes won't find new iPhone and update invalid signature

Question

Trying to restore from iPhone 4 Restore backup to new iPhone 6 (Windows 7) and encountered the following......
When trying to connect the new iPhone 6 to my existing iTunes account, iTunes would not show the device (couldn't find it). Received a popup that stated the phone could not be recognized and an update to 11.4 (12.1....) was required. [[ I found this strange since I just updated and backed up my iPad and old iPhone 4 last night in anticipation of getting the new iPhone 6 today.]]
I tried the update multiple times but it would stall 15 minutes in and give the message "invalid signature" and error out.  I looked through many comments on this support sight, read what related help was available through iTunes help, etc, and spent HOURS trying each one (listed below).  A combination of a couple of steps finally worked for me so I am sharing this to hopefully save you some time and frustration. MUCH frustration.
Tried but did not resolve..... updated new phone to current 8.1.1 and made sure 'Find My Phone' was turned off.
Tried but did not resolve..... downloaded iTunes manually from within iTunes.
Tried but did not resolve .... downloaded iTunes manually from Apple.com downloads.
Tried but did not resolve..... downloaded Firefox (an alternative browser) and manually downloaded iTunes again from Apple.com.
WARNING: The following resolution will wipeout any previous iTunes settings like views, organization, etc.  It will also wipeout any previous setup already done in your new iPhone other than fingerprinting, language and date/time setup. Email accounts, setup preferences, etc, will be wiped.  Also, your old phone will no longer work with iTunes - apprently this new software issue will only recognize one phone.  Your music, contacts, most apps (I did lose a few), if backed up from your old phone properly before starting this process will 'Restore' to your new phone in the updated iTunes.
Tried and worked..... Closed iTunes. Went to Start on my computer, typed itunes64setup. I had two itunes64setup listed due to the first manual download from Apple and then the one done again via the Firefox browser.  I chose the one downloaded from the Firefox browser - not sure it would make a difference. (If you have two listed, Hover your mouse over each and it will give you the times of when they were downloaded).  Right click on the itunes64setup and choose 'Run as Administrator'.  (You may incur issues with this step if using a work proprietary computer.)  Opened iTunes and plugged the iPhone 6 in and it recognized the phone. I was then able to 'Restore' info.
Since I had already tried the manual downloads, I am not sure if they were necessary and maybe running the itunes64setup only would have resolved the issue.  If you are addressing the iTunes update iPhone 6 issue and try the itune64setup without the manual downloads, etc, and it works, please update this support entry so others can follow your lead.
Thanks!
What Apple is not telling (WARNING) their faithful users about is that they obviously have software issues for existing users in the 'Windows' world when upgrading to the newer phones and to do so you are going to need to spend some time and effort.  This is strike two for me, just coming off the 8.1.0 iPad 2 upgrade debacle Apple put so many of us through.  Shame Shame, Apple!  It's called Q/A and should be applied to all your users environments and not just those who have everything new in an Apple/Mac only world. There are more of us that don't. Thx!

Answer 1

Step one : Please restart EVERYTHING involved. This means your cable modem / dsl router, Airport Base Station if you have one, your computer, your iPhone / iPod Touch / iPad (2) if applicable. Shut it all down and power it all back up in the appropriate order (modem / router / computer, et cetera)
Step two : Make sure you have the most recent version of all software. Verify you have the latest version of iTunes and the Remote software. Check to make sure the AppleTV has no updates as well. 
Step three : Enable Home Sharing on all applicable devices.
Step four : Make sure the AppleTV and the computer are both hooked up to the same network, wireless or otherwise, and make sure if you have a non-Apple router anywhere in the mix it is allowing traffic to flow freely between all the Apple tech. Apple has specific documentation on this. Search the Support site for "Apple TV: TCP and UDP ports and protocols used" and make sure none of those ports are being blocked by the router or any firewall software you may have running, like Zone Alarm.
Step five : Fire up the Remote app on your phone, and let it search for your AppleTV. There should be no passwords required anywhere if the Apple tech is properly talking to one another.
Step six : If all the above still doesn't work, then call in a professional who can get eyes on the equipment and do a proper job of troubleshooting.
Please let us know if this does not help.