Users using default administrator account blocked from apps in Windows 10
Microsoft has been improving the security of the operating system bit by bit with each release. This is great news in the long run, but can catch some users who had poor security configurations in a bind when they upgrade. For example, Windows 7 allowed the default administrator account to be used by local users. Windows 10 forbids many apps from running under the default administrator account, which is a good idea as the default administrator account should be used for... administration.
Still, we have millions of people upgrading to Windows 10 it was inevitable that some would be running under the default administrator account and then upgrade. The correct fix for this is to create a new local user account, but those same users who upgraded in such configurations also have a lot of software that has been installed over the years that they want to "just work" after such an upgrade.
The fix is actually quite easy, if not recommended. As the impacted user on Windows 10, first, go to the Local Security Policy control panel. Navigate Local Policies, Security Options, User Account Control: Admin Approve Mode for the Built-in Administrator Account and Enable this setting. Then in the registry, \HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\UIPI (default) which should be set to 0x00000001 (1).
With this in place, restart the machine. Microsoft Office and other apps that Windows 10 would otherwise prevent from running will now work. We are forgoing the improved security model, but this configuration is no worse than the Windows 7 security they came from. Still, I recommend taking the time purchased with this workaround to set up a non-default admin account for the user.