You cannot log on after you remove the computer from the domain
After removing a computer from a domain you cannot login and receive the follow error :
This is due to the previous credentials used were only for authentication to that of the domain. Once your PC has been placed into a workgroup you will need to use the username and password of the local administrator that was configured when Windows was first installed.
But I dont know the Password
If you find yourself in a situation where you do not know the original local admin credentials, then you can use Hirens Boot CD to clear the password. This will allow you to log into windows without a password, once in you can then set yourself a new password.
Further details can be found at http://support.microsoft.com/kb/317049