There is always at least one administrator account on the computer,
including any you may have created and named as well as the original and
invisible one that is always there. To access the default administrator
account, log out of any accounts, hit Ctrl Alt Del, and in the dialogue box
that appears, for account name type 'administrator' and the password if you
have given it one. From here you can create another account for yourself
and give it administrator status. Do this by hitting Start Settings Control
Panel User Accounts. You will now be able to change things from within
your own named account in future when you log in.