I would change the local Admin password by using the chntpw utility.
Having changed from the domain to the workgroup it is now assuming the user is a local user and not a domain user. try putting your domain infront of you login name domain/name (or is it domain\name) try both.
There are several boot CDs out there that will allow you to change passwords – your IT dept should be able to point you in the right direction.
Using domain\username won’t work now as it’s not a member of a domain. Likewise, you won’t have the option to choose a domain in the logon box either as Local Machine is the only option. When you finally logon, if you like, you can revert back to the fluffy logon buttons and fast user switching from the User Accounts section in the Control Panel.