This question already has an answer here:
I have exactly the same problem as was already described (by Rajendra Rathore): "... I removed my user account from administrators group and my default administrator account was disabled by default. So now i am unable to perform any administrative task. Is there any way to add my user account in administrator group OR Enable default administrator account in windows 7"
I heard that it could be fixed by Hiren Boot CD - I have it but I don't know how to use it. Could you kindly describe the whole procedure step by step.