I was suffering from the same problem but i found solution for administrator login from the following site:
Enable the (Hidden) Administrator Account on Windows Vista :: the How-To Geek
The details are as below:
Enable Built-in Administrator Account
First you'll need to open a command prompt in administrator mode by right-clicking and choosing "Run as administrator" (or use the Ctrl+Shift+Enter shortcut from the search box - The easiest way is to use the type
cmd in the
Vista Start Run box and use the
keyboard shortcut Ctrl + Shift + Enter instead of just pressing Enter. This will open the
Command Prompt in Administrator mode.)
[IMG]file:///C:/DOCUME%7E1/MFD1234/LOCALS%7E1/Temp/msoclip1/03/clip_image002.gif[/IMG]
Now type the following command:
net user administrator /active:yes
[IMG]file:///C:/DOCUME%7E1/MFD1234/LOCALS%7E1/Temp/msoclip1/03/clip_image004.gif[/IMG]You should see a message that the command completed successfully. Log out, and you'll now see the Administrator account as a choice.
[IMG]file:///C:/DOCUME%7E1/MFD1234/LOCALS%7E1/Temp/msoclip1/03/clip_image006.gif[/IMG]
You'll note that there's no password for this account, so if you want to leave it enabled you should change the password.
Disable Built-in Administrator Account
Make sure you are logged on as your regular user account, and then open an administrator mode command prompt as above. Type the following command:
net user administrator /active:no
[IMG]file:///C:/DOCUME%7E1/MFD1234/LOCALS%7E1/Temp/msoclip1/03/clip_image008.gif[/IMG]
The administrator account will now be disabled, and shouldn't show up on the login screen anymore.