Networking problem with Windows Millenium and Windows XP SP2

Hello
I have make a local network through a modem. I have 2 machines, one with
W.Millenium and the other with W. XP Sp2. I´ll shared on both the entire disk
with full control.
The problem is i can through XP i open all the folders and files that i have
on W.Millenium machine, but the inverse don´t work.
Through the W.Millenium machine i find the computer, i can enter on it and i
saw the shared folders, but if i try to enter on any one the computer send me
an error message " session not iniciate".
It´s seems to me that when the machine are started they cannot enter on
session.
I don´t know how to resolve this problem because i need full access on both
machines.
Could you give me some help.
Many thanks
Luis Santos