Well then you need to go back to whoever you bought it from and get the installation disks, and/or get the previous owners log in details (you can clean up the settings to your own preferred settings and remove the account of the previous owner and add yourself once you can log in).
You might also try logging in with administrator as the username and a blank password - this might allow you to bypass the current user login and when you are logged in as administrator you can change the settings to remove the previous user and add an account for yourself.
If none of the above work - you need to get a windows installation disk and reformat the hard drive and install windows again. I have no idea how expensive a copy of windows is or what version you would want but you can google that information.