VPN for MS XP professional

I have a VPN going to connect 2 machines: a Windows XP
machine and a Windows 2000 machine. The windows XP
machine can open a share from the windows 2000 machine,
but the 2000 machine tries to open a share it gets a
login failu unknown username or bad password. We have
tried using multiple proper usernames and passwords still
to no avail. If memory serves there is a program to run
on the 2000 machine so it will talk to the XP machine.
Does anyone know this mystery program?? Help!!!