User Accounts Administrator Change?

Okay so this is my problem; About two weeks ago my mother got to keep a laptop from work because she was moving schools and it was "outdated".It was on a school domain so it had her login from the schools.At the login page all i had to do was type "name.name" and the password and that was fine. She did not use the laptop anymore so she gave it to me.Even though she was a administrator i was getting sick of logining in as her all the time so i tried to create a new account. I want in to controle pannel and tried to make one but it came up with domain and user name. i tried to create a account but it was saying the rustrelationship with this domain and Lt-D-8JK12S. (the laptop) were denied.I went through another way to create a account through some workgroup thing , it worked. When i had to log on i would have to type Lt-D-8Qjk12Susername and password It only happened about two days ago that it started to have some problems.I could only log on to my account even though it had many different users in the laptops hard drive. it would say that i got my password wrong when i tried to type.the main problem is that i cant do anything because i am not an administrator and i kept being denied when trying making system changes and copy files on to a memory stick.