Do you mean Windows NT 4.0?
GetLongPathName is not supported on NT 4.0, see MSDN:
Client: Included in Windows XP, Windows 2000 Professional, Windows Me, and Windows 98.
Server: Included in Windows Server 2003 and Windows 2000 Server.
A workaround (also from msdn):
Windows NT and Windows 95: Include an additional header file called NewAPIs.h to make GetLongPathName available on these operating systems. The function is not implemented natively, but by a wrapper that utilizes other native functions on these systems. See the header file for details of the use of preprocessor directives that make the function available. If you do not have this header file, it can be obtained by downloading the most recent SDK from the SDK Update Site.