Can user protect his files from untrusted administrator? [closed]

Your first problem is you have an administrator that you don't trust. This is a human resources issue, and you may need to take it up as such.

Second, you could only "protect" them by placing the files in a volume that's encrypted separate from Windows (encapsulating the data). There are methods to do this and products to do so if you google for it.

BUT

Do you own this material? Or is it company owned? Are you in effect stealing from the company?

And beyond that, this material could still be copied, and the computer could be monitored via keystroke capture and monitored remotely through screen capture/viewer programs, both of which are usually allowed in most countries because in the workplace, your employer owns the computer and the network.

You need to take this up with your employer. The sysadmin has full access to the computer, and the employer owns the resources. Whatever your issue is with the sysadmin you need to work that out without a technology solution.

Nothing would prevent the admin to install keyloggers etc., and you wouldn't even notice, so: No, your assumptions are wrong.

An "untrusted" administrator should be getting his rights revoked. There are tons of other ways to steal data if you have full rights on a server.