Richard was close (or on target if reading correctly)

The Windows 9x network redirector DOES NOT support you supplying alternate credentials.

NOT SUPPORTED
Use y: "\\myserver\myshare" /user:mydomain\myusername /password:secret

If the Workstation is in the same DOMAIN and the UserCredentials match those on the Domain Account it should map.

On a Workgroup it's a bit different, but I'm assuming you're trying to connect to an NT Domain share point.