I think DS9 is on the right track, but I'll offer a little more...
I gather that you are trying to map a drive on the workstation, to the server. If this is the case, you have to make sure that on your server you have shared the particular driveor folder, and assigned the appropriate permissions. The permissions part is important, as Windows NT employs the most restrictive permission set within a tree of folders.
Also in Server Manager, check out your server - there should be a Shared Directories menu option, which lists all your shares. By default, all installed drives will show up in the Shared Directories list as $ - which is a System and Administrator share. A Windows 9x machine will not typically be able to map a drive letter to C$ - therefore you need to actually share the drive, or create a folder and share it.
For example, on your server's C:\ drive you create a folder called \ALLDATA, and you share the folder. You set permissions for the \ALLDATA folder for the Everyone global group to have read & write access. Then you can go to the client PC, log in to the domain, browse through network neighborhood to the \\\ALLDATA folder and map a drive.
Good luck!