Just received the first update to Windows 10 today and it failed to install.

I found a solution on another forum.

remove SID’s for users that no longer exist in the registry. 

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

you can check which users don’t exist by checking computer management/local users and groups/users.

this worked for me on both of my windows 10 PC’s, best to be safe though and backup your registry.