Sounds to me like it's just a simple problem of not having the right NetBIOS username/password. This isn't a Ghost problem, it's a DOS-level networking/file-sharing problem still.
I don't know what the server's OS is (the //computername/ that you're trying to mount a volume from.) But make sure that server is accepting LM based authentication. Many places have moved to accepting only NTLM (or even only NTLMv2) and that won't work with DOS-level file-sharing usually.
Make sure your DNS name and IP address and all that matches your server's NetBIOS computername. Way too often I see people set their computer's name to one thing, but the IP address the machine gets has a different DNS name (forward and reverse) attached to it. It shouldn't matter at the DOS/NetBEUI/WINS level but you can never tell. I don't think this is it, but it's worth a look.
Can you succesfully mount //computername/foldername from another computer that's just running Windows XP? Skip DOS for a moment, and verify that the exported share actually works from another "normal" machine. Since you're using PXE, I'm guessing your target Ghost environment is DOS. You mentioned "before the machine goes to Ghost32.exe" so I wasn't sure if you were booting into Windows PE using 32bit Ghost32.exe, or if you were PXE'ing into DOS and using 16bit DOS Ghost.exe
PH