Bad sectors are places on the hard disk which are no more fit for storing data. Hence no data is written on to them. The bad sectors may spread when you format your hard disk often.
To identify Bad sectors you will have to perform the check disk feature in windows.
Just right click on to the drive which you want to check and then click properties.
Navigate to the tools tab and then under the Error checking label, and click on to the check now button.
You will get the Check Local disk Dialog box.
Just check on the scan for and attempt recovery of bad sectors check box.
Then press the start button.