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 /active:yes" (Note the capital "A" in Administrator) and press Enter, you will get a confirmation as "The command completed successfully".
3- Click Start, and type "regedit" in the search area and click Enter, navigate to: [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]
Double click on "FilterAdministratorToken" and set it to "0"
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Now log-off, and you'll see new account named "Administrator" is available, click on it to login.
Now you are the master of your domain! I recommend if you're going to use this method is to apply it as soon as you do a fresh install of Windows, so you can simply delete whatever administrator you've created in the setup process, and make this one the "real" administrator for your PC, also you can rename this new admin account or change its password like any other account from "User Accounts" in the Control Panel.
A last note/disclaimer:
Please note that disabling UAC and using the built in Adminstrator account will also disable IE7 "Protected Mode"