I would hasten to add further to elbanditoroso’s response that it is highly ill-advised to leave the hidden administrator’s account active! It’s hidden for a very good reason. And that reason is to prevent people from doing things they shouldn’t.
To that end, when you’ve done what you need to do – and I must stress you better be darn sure you know exactly what you’re doing, if you fudge it in that account you are looking at one broke computer, you MUST hide the hidden account once more.
In order to do that, you should use the method described above, and in the DOS box type
net user administrator /active:no
And to be certain…make sure you know what you’re doing before you do it!