It depends on how you, the Administrator, have setup your files, and how you have set up their user permissions. As the administrator and system owner it is your responsibility to make sure that your system is set up in such a way that it is safe for you if they use it.
However if there are things you absolutely must keep confidential, those files should NOT be on that computer. They should be encrypted and on something only you can access(encrypted, password protected USB drive or a secure off site location). You should then probably avoid opening them on that computer if you're really that paranoid about them doing something to try and see them.
Remember if you keep that computer as safe as you're willing to have it, then he can only access what you're willing to let him access. There are plenty of tools and techniques to do so, but it is ultimately up to you to put those practices in place.
If you just want to keep them hidden you can encrypt the files. If you want him to not even be able to read you folder you need to setup those folders for your access only. If you don't want him to launch an application you need to limit user rights to that application. If you want.... That list goes on and on forever, but you get the idea. It's all pretty much up to you.
Here is another question you should probably ask yourself:
If there are things you don't want guest users on that computer to see, why are you letting them on that computer?
After all physical access is the greatest threat to any computer.