Though it is an old post, but I ran into same problem on my recent update to windows 10, I couldn't find solution to this while looking on internet, but I manage to solve the problem, So I thought to mention it here so it could save time to someone who ran
into such trouble.
while we upgrade to windows 10, the upgrade actually creates a backup registry entry of your profile which you used for upgrade and ran into problems.
Note; Before you start, please make sure to take back of your registry key, and do not attempt if you are not familiar with changing registry entries, and finally if you do so, you do it at your own risk. I will not be held responsible for any loss or damages.
If you go to following registry key...
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
Under this registry key, you will find keys listed of profiles of any number of user you have on that machine, the one will be with ".bak" extenstion, that is actually the backup of your profile that was working fine before update.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\S-1-5-21-979814014-2414503015-3075406181-1001.bak
now you need to look for the similar key but without ".bak" that the update wizard has created but could not log into.
which should be...
S-1-5-21-979814014-2414503015-3075406181-1001
Now what you need to do is simply rename this key to something else I say just add ".bak1" to its end, And finally remove the ".bak" extenstion from ".bak" the backup key.
Close the all open windows.
Restart your PC, It will restart again as if it just has finished upgrade, and will display same messages like "we making some interesting changes to you pc" etc. but as it finishes, you will have your desktop back with all settings as it was before update.
hope it helps.