I think it may have something to do with SMB signing. I'm in a Windows 2000 Native-mode Active Directory domain. The client is able to map a drive to a Windows 2000 server without issue. We recently added a Windows 2003 server, and moved data around. User still needs access to the data that's now on the 2003 box.
Yea, I know--should upgrade the PC from Win98 to XP, but we no longer have the software that's required (and is installed on this PC)... so upgrading the PC isn't an option at this point.