Problem in a system due to password at login:

The problem in the system at the login of the system will be the password.Some of them will forget their login password.So due to this problem they are not able to login in their system.Without login we are not able to operate in the system.One of the method which i know to rectify this problem is to reinstall the OS(Operating System).While we are reinstalling the Operating System we are able to rectify this password problem.While we are reinstalling the Operating System,the datas we had saved earlier in the space where we had installed the OS will get erased.So an important information to all of them who are using the system is better always store the important datas in the other colon except the colon which we are installing the Operating System.