when you first click on 'check now' to perform scan a screen opens with two check-boxes.
Automatically fix file system errors and
scan for and attempt recovery of bad sectors.
The first fixes 'normal' errors, the second causes every sector of the disk to be read and tested. takes absolutely ages, but is necessary if you get ANY errors from the first check.
If events cause you to suspect your hard disk is failing, then go to the manufacturers site and download their own professional test suit.
in reality, hard drives by certain manufacturers are less reliable than others, and can fail after two to three years. Others can go on for ten years or more. In my experience, drives USED to be more reliable than they are today - obviously this is due to the many improvements designed to increase area density and response time, but have the inevitable side-effect of calling for MUCH higher tolerances which cannot actually be sustained for all that long.