Windows XP Pro workstation would not access Windows 2000 server shares. I think the error mentioned "invalid username" even though both systems are on a Windows domain and the user in question is a domain user. All other network connections to other servers appear fine.
Turns out, the workstation had somehow saved a different password specifically for this server's connection. Removed it by going through Control Panel, User Accounts, Advanced, and Manage Passwords.
Here are some sample images of this bizarre area:
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