I updated my 800 over 2 weeks ago. It was not a simple update at all and kinda a PITA. I did NOT use NaviFirm, but I had an unlocked one that Microsoft sent me and I had taken it off all of my accounts so I could not get an update from a carrier. Like I said, this is a pain, but worked for me. It actually updated the phone 3 times, so I had to do these steps for each time. My laptop that I did this on is runing Win8 x64, not that it should matter. 1. Turn off all wifi and cell signals on the phone. You can put it in airplane or just turn those things off. 2. Connect the phone to the computer and run the Zune software. 3. Go to the Settings -> Update section. 4. This is the painful part... hit the button to check for updates, after about 3 seconds disconnect your machine from the internet. If you get an error that says that it cannot check for updates, then reconnect and try again. You also could get it to say that there are no updates. Again, reconnect to the internet and try again. This took me about 10 times to get it to show the update. 5. When the screen shows that there is an update, reconnect to the internet before hitting the continue. 6. After it is complete, check your version to see if you have the latest one. Like I said above, I had to do these steps for 3 separate updates to get all the way to 7.8. If you are on a carrier, it might be easier, but I know that this is how I updated my Lumia 800 up to 7.8. Good Luck.
Steve Maier