The best way is by personal recommendation. If you have any particularly sensitive files, I'd save them to an external hard drive, then delete them from the PC. From then on, only save sensitive documents to the external drive. Then, if it goes wrong in the future, not only is the technician unable to see anything you don't want him to, but also your documents are safe.
Amazon Fire HD 8 review: A brilliant combination of function and value – with one massive caveat