The administrator account is disabled by default in Windows Vista, here is how you can enable it at logon.

Ok there are really two ways of doing this depending on which version of vista you have. And in 1 or 6 easy steps!

NOTE: Editing the Registry incorrectly can cause serious, system-wide problems that may require you to re-install your operating system to correct them. Microsoft nor computingondemand.com cannot guarantee that any problems resulting from the use of registry editing can be solved. Back up your registry first, but use at your own risk.

Method 1: All versions of Vista

1. Open the registry editor (Run as Administrator)
2. Navigate to the following registry entry: HKEY_LOCAL_MACHINE/Software/Microsoft/Windows NT/CurrentVersion/Winlogon/SpecialAccounts/UserList
3. Look for an entry on the right side for Administrator. If it doesn’t exist, you will need to create it. To do so, right-click on UserList and select New > DWORD Value. Give the value a name of Administrator. Press the Enter key when you are done.
4. Right-click the Administratorentry and click Modify.
5. Set the Value data to 1 and click OK.
6. Close the registry editor and restart the computer. The Administrator account should now be an option on the login screen.

Method 2: Vista Ultimate and maybe business

Go to C:\windows\system32 Once in that folder, right-click on lusrmgr.msc (easier if you search for *.msc) and select run as administrator. Now select the users folder, you'll see all the user account once in that folder, right-click on administrator and select properties, uncheck 'Account Is Disabled', restart and your administrator account should be enabled at login. You can also set a new password here in case you forgot or you have a pre installed version (aka OEM/System Manufacturer) and do not get to choose a password.

lusrmgr is disabled in Vista HomeBasic/Home Premium.

Method 3: Using a Command Prompt - Probably the safest way (Joe DiFiglia)

1. Open an elevated command prompt (right click on command prompt and select "run as administrator")
2. type: net user administrator /active:yes

To disable this account after you perform whatever tasks you may need to use the admin account for

1. Open an elevated command prompt (right click on command prompt and select "run as administrator")
2. type: net user administrator /active:no

NOTE: Make sure that you set a password for this account if you decide to leave it active.  It does not have an associated password with it by default.

I know there are a few other methods for accomplishing this, Junior has pointed his methods and I mine... Please comment with your own methods!