How to create a undeletable registry key (or file) in C++

Question

I would like to create time limited version of a game I am developping in C++. I already meet some program that create files I am still not able to delete, and other that created registry key very hard to delete (because there were several sub key and needed to change owner to the deeper first, delete it go up, change owner...).

Then I think the only way is to create something in the computer that is not (or hardly) deletable to mark the beginning of trial. I know how to create key or file but not how to protect them.

Thanks.