log on

I just bought a new PC after going to Cyber Warehouse and having them tell me that my pc would need a replacement motherboard because mine stoped working. He told me that if I got a new motherboard that all my parts would need to be replaced because they don't make my motherboard any more and my parts would not connect to a new motherboard and that it would be cheaper just to buy a new one and that the new one would have more memory, better graphics, and DVD/CD burn and so on. So I bought it and it works great much better, but I am having a problem. I only have one account and no password so my two sisters, my step sister, step brother, mom, and step father can all get on. The PC is a HP and even though I only have one account and no password it keeps going to the log on screen says HP_Administrator as the user. It is a different account then the log on account I made. I there away to have the PC sign start on to are family account without going to the log on screen and having it stop saying HP_Administartor as the account. I cant still sign on under the family account but I want to know how to make the Admin account go away. I think the family account set for admin.