s there someone who works at or has worked at a computer repair centre who can shed some light to this? I imagine a format is done to a computer first to ensure that the problem is hardware related and not software; but what is done to actually find out which component of the computer is responsible for a problem if there are no errors or anything present?

Say for example a computer is running slowly, but after a format, it continues to run slowly. The RAM and processor appear to be running smoothly, but yet a computer tech always finds out what the problem is, even if the usual problems are eliminated. How would one go about testing? Are special diagnostic tools used? If so, what are the most common ones? If not, what is done?