Delayed reply as the ISO took an absolute age to download!
All installed and updated, same problem exists.

I keep getting messages that apps "cannot be opened with the Built in Administrator...

I'm reluctant to lose the software that is installed. I there any way that I can try to reset without losing this?

No I hadn't, but I have now and same error. I'll follow your link and see if that helps.

Thanks.

Hi, I'm new to Windows ten having been coerced into "upgrading" by Microsoft!

I only seem to have one account, as "Built in Administrator" but can't add another account or sign in using Microsoft...