Computers have locks just like your home's front door and their purpose is to keep people out. A login password will not keep many people out--it is easy to gain access to unencrypted data. On computers running Windows OS there is a free program called Ophcrack. The program is based on Slackware and uses rainbow tables to solve passwords that are up to 14 characters in length. It takes approximately 10 seconds to find the password and there is little to no skill needed to perform this.
The How To
Download Ophcrack and burn it onto a blank CD or load it onto a USB via UNetbootin. Then insert the CD into the computer and turn off the computer (by holding the power button until it shuts down). Turn it back on and enter the BIOS at the startup, change the boot sequence so that CD (or USB) comes before HDD, then save and exit. Restart the computer and Ophcrack will start to be loaded. Let it run, write the down the passwords it gives, remove the disc, restart the computer and enter the password given by Ophcrack.
Difficult Passwords to Hack
If you're worried about someone hacking your computer with a program like this or someone else hacking you over the internet, check out my article on creating a safe and secure password that is easy to remember.
Hope this helps!