Multiple Users back to Administrator User

OK My computer has been running under the Administrator
User since I've owned it. But then someone decided to
mess with my computer and add another user. Well when
they did this I can no longer get into the Administrator
user profile. What I want to do is put it back to only
having the Administrator profile and no other users on
the computer. I've been told this isn't possible. But the
person wasn't totally sure. Can I do this? And if so how?