This may be useful if you've installed Win10, but need to quickly get back to a usable machine without spending an entire weekend re-paving it!
I installed (upgrade keeping files/apps rather than clean install) Windows 10 on my work machine on Thursday night last week, then found on Friday my existing VPN connection failed with a message like "Application not found" and the OS went off to the Windows Store to look for my VPN!
I deleted the existing VPN connection and created a new one, only to receive "A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provider" - I've seen this issue before on my machine after a crash and Windows not shutting down properly - I managed to fix it by rolling back to a system restore point, however the machine is on a new operating system!
Luckily, there is an option in Updates & Security/recovery to "Go back to Windows 8.1" (or whatever OS it came from) this option is available for month as lone as long as disk cleanup utility hasn't been run to remove previous installation files. I returned the machine to 8.1 which took about 1/2 an hour and started fine, although I still got the root certificate error again.
I was preparing myself to spend all weekend re-paving my machine, but remembered the problem I'd had previously and looked in System Restore (Type recovery on Win 8.1 start screen, then go to "Open System Restore"). Luckily there we still some restore points and I was able to restore to a point just before installing Windows 10.
After another hour or so, the OS was restored to before Windows 10 and my VPN connected fine and I was able to work again!