How to return a computer to factory settings?

Hello,

I bought a gateway computer at a yard sale a few days ago only to find out when I got home that the system is password protected! It's a windows vista system. According to the guy that sold me the system, I have to original discs and paperwork that came with the computer. I need to know how to reset the computer or remove the passwords or what ever I need to do to reset it to factory settings. I'm not very tech savvy so things may have to be spelled out for me :P Thanks!