After installing a Windows 2003 Server and trying to mount a share in Mac OS X, everything was fine in Workgroup mode. However, after putting the Windows server 2003 in Domain Controler mode, I started to have get -5000 errors coming from SMB. The Macintosh File Server is installed.
After some searching, I found that after promoting a Windows Server 2003 to a Domain Controller, you must check the Domain Security Policy and de-activate the "Always secured connection" policy. I don't understand why, but after changing that setting, it works fine.
Mac OS X Hints