You need to restore the operating system to its factory pre-loaded state. Your computer will then be as a good as new.
This is easy.
If you have any files on the computer that you want to keep, put them on a CD or memory stick.
Some computers have a recovery CD, others have a recovery section on the hard drive.
If it's a CD, you put the CD in, restart the computer, and follow the instructions. If its on the hard drive then you use a certain key combination during startup. If you tell us which computer you have, we will be able to tell you.