Password unknown

I bought a computer at a yard sale and was told that I would have to reinstall Windows XP on it. When I turned the computer on, XP is still on it but it requires a password to enter. It says to hit control, alt, delete but when I do, the same window comes up again. I hit F8 as Windows was opening so I could go into safe mode. When I go into safe mode I get the same thing. Hit control, alt, delete. I don't know who I got the computer from so I have no way of getting a password. Can anyone help? I don't know very much about puters at all. Thanks in advance.