After removing a computer from a domain you cannot login and receive the follow error :

The system could not log you on. Make sure your user name and domain are correct, then type your password again. Letters in passwords must be typed using the correct case. Make sure that Caps Lock is not accidentally on.

Why ?

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