How to "protect" the pc when it is in assistance?

Let me explain. Let's pretend that my pc doesn't work anymore but it's still under warranty. I give my pc to the store to fix it.

Now, if I had photoes or sensitive data on my pc, how can I be sure that none is going to watch or save them while trying to fix it?? Is there a tool that helps me to keep my sensitive data secure?

privacy protect