They may have enabled the built-in Administrator account that is not the same as the one you set up while installing Vista. I would not recommend
deleting this account though it can be easily hidden once again.
Deleting that account will leave you with no way to repair your computer if something should ever happen to your only default administrator account that you had set up during the installation of Vista.
If you have your UAC (User Account Control) turned off, please go to Control Panel > User Accounts and turn it back on. This needs to be accomplished with an elevated command.
If you have never turned it off, please proceed as directed:
Reboot your computer and make sure that you are logged into YOUR
Administrators account that you created when Vista was installed.
Go to Start > All Programs > Accessories > and right click on Command Prompt and choose Run as Administrator
from the list. Click Continue
in the User Account Control
window that pops up. In the Command Prompt window type (or copy and paste) the following command and press your Enter
key:net user administrator /active:no
You will get the message, The command completed successfully.
If not, try it again.
Reboot your computer once you see the message and you should only see your Account once again. If you have any trouble let me know and I'd be more then happy to guide you further.