In fact the account "Administrator" is disabled by default and we can take

the following steps to enable it:

1. Click Start and right-click Computer to select Manage.

2. Please click Continue when you are prompted.

3. Expand to System Tools -> Local Users and Groups -> Users.

4. Double-click Administrator to open its properties.

5. In the General tab, uncheck "the account is disabled".

6. Click OK then it is enabled again.

Additional information

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Windows Vista™ makes a distinction between the built-in administrator

account and members of the administrators group. The built-in administrator

account has full read/write access to the computer and runs with the full

administrative access token. The built-in administrator never receives a

split token. Users who are also members of the local administrators group

run with a filtered token, with the same privileges as a standard user

account. Only when a member of the local administrators group gives

approval can a process use the administrator's full access token.