Activate Administrator Account in Vista

For Windows Vista Ultimate/Business/Enterprise:

1- Click Start, and type "secpol.msc" in the search area and click Enter. (You may receive a prompt from UAC, approve/login and proceed)
2- In the left list, choose "Local Policies", then "Security Options"
3- Set "Accounts: Administrator account status" to Enabled.
4- Set "User Account Control: Admin Approval Mode for the Built-in Administrator account" to Disabled.

For Windows Vista Home Basic/Home Premium:

1- Click Start, and type "cmd" in the search area, right click on "Command Prompt" and select 'Run as Administrator".

2- In the command prompt type
"net users Administrator password" (Type your own password)
"net users Administrator /active:yes"
(Note the capital "A" in Administrator) press Enter after each command and you will get a confirmation as "The command completed successfully".

Now log-off, and you'll see new account named "Administrator" is available, click on it to login as Administrator and your password.