The username for your domain, as far as Windows is concerned locally, may not be what you think it is. The most reliable way to deal with this:
- Start the advanced user accounts control panel applet by e.g. typing "control userpasswords2" in the Run... dialog (XP) or the search box (Vista).
- Click the Advanced tab.
- Click the Manage Passwords button.
- If your server is listed here, select it and click Remove.
- Click Add.
- Enter your server name, the name of a user on your server (in the format <servername>\<username>, e.g. rick_server\rick), and the user's password.
- Confirm your way back out.
This will cache a set of credentials locally for your server, and associate them with your domain account.
I'm not on the WHS team, I just post a lot. :)