Data and files stored on your hard disk may contain a very sensitive information , previous study conducted by Researchers from BT and the University of Glamorgan demonstrated at which point reputable organization do not care about data security.
Researchers started by purchasing 300 hard drives previously used from online store (eBay) and they recovered HD data with some standard tools to find users banking information, security logs , Business deals and even details about test launch procedures for the THAAD ground-to-air missile defense system.
First of all you need to encrypt your sensitive information with a solid encryption tool and when you are going to sell or give your computer to any person be sure to wipe the data with utility such as Darik’s Boot and Nuke (“DBAN”), DBAN is freely available and will delete your data to not allow other utility recover your content.
DBAN is an iso image that you can burn to CD or USB stick and erase all data and even partition table on the hard drive, you will notice that after wiping your HD installing any operating system will be as you just purchased the computer and make you even confident that there are no malware in boot sector ,so even if you will reinstall a system at the office level you can use it to be sure that no one can recover any file on the HD.
You can read more and download this tool at the official website: http://www.dban.org/