Right click the Windows (Start) icon and go to Properties. Start
Menu/Customize. Make sure that "System Administrative Tools"
is checked to display.
Go to it and click Computer Management.  Then double-click Local Users &
groups.
Right-click Administrator in "Users" - Properties, and uncheck the disabled
line.  Do NOT apply a password whilst in that module!
If you want Administrator to show on your regular Welcome screen then do the
following:

Show Admin on Vista Welcome Screen

Show Administrator on Welcome Screen in Windows Vista

start -> run = ” control userpasswords2 ” select administrator account,
click reset password (input new pass) / Ok

start -> run = ” regedit ” create the key below

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows
NT\CurrentVersion\Winlogon\SpecialAccounts\UserList

create “DWORD (32bit) Value”
Value Name: Administrator
Value Data: 1

http://blogs.msdn.com/windowsvistasecurity/archive/2006/08/27/windowsvistasecuri
ty_.aspx
Here is your reason why the Admin account is disabled.
 Why is the Admin account blocked. Having screwed up video drivers, I tried to
 roll back (grayed out), and then boot to safe mode only to find that the
 admin account was disabled