Sounds like you're stuck trying to authenticate to a Windows domain instead of the local accounts. From the login screen, see if there is a Log on to: dropdown (it may be hidden by the Options button), in there there should be an option called: XXXXXXX (this computer). Select that one and try the user Administrator. If you created a password during install use that, if not try it with a blank password.
If that lets you into the Administrator account, you can switch your computer from Domain to WORKGROUP:
- Right click on My Computer, click Properties
- Click Computer Name tab, then the Change... button at the bottom.
- Switch the Member of selection to workgroup and if you don't use one already on your LAN just call it WORKGROUP or MSHOME
- Reboot and there should be a normal login prompt..