destinct

Hi, I bought a second hand laptop for my sister, It was by a business before I got it and when logging in asked for username, password and domain. I wanted to get the computer back to a normal home pc so I went through a network settings wizard and selected that it was a home pc and not used in a business so it didnt ask for a domain ect, It then told me it needed to be restared to apply these settings, when it rebooted I couldnt log in! I get an error message saying the username/password/domain is incorrect, I cant enter a domain anymore because it doesnt come up in the options on the log in! I dont know what to do!! could I use a restore disc from bootup? ive tried safe mode ect but it still asks for a login and says its incorrect! help please! I wanted to give it to her this week! note, its running windows xp professional Joe