You just got that great deal on a used computer from work, or from a surplus sale. You get home turn it on and you need a password. the user name is someone you have never heard of and every thing you try fails. You change the name to administrator and it is the same. well your in luck, I have found an Emergency boot CD that will solve this problem, for windows anyway. You have to download the files and make an ISO, or CD image, file that you can burn to a CD using your CD software or download the demo version of Nero. Naturally you will have to do this on a machine you have access to.
Once you have made the CD you will boot the problem machine from CD and choose the password utility. This CD is based on a small Linux kernel and you will see many other utilities that come with the program. Read the screens as you go and go slowly. When I tried this it work well and you select the defaults along the way. Clear the password rather than change it.