samba - Jun 2004 - why doesn't Samba Server want my password?

hello,
I've got a strange problem here. i set up a samba server on a trustix
linux system, everything works fine except for 

the fact that a user on a windows machine can log on to his account on
the server without having to give a password. 

He's logged on on the Winxp pc with his username(same as on the server)
but without a password. 

i don't understand what's going on, I guess it's something quite
trivial
but so far neither google nor the howtos I've read have given me an
answer, hope someone can help me.



here's my smb.conf:

# Samba config file created using SWAT
# from 0.0.0.0 (0.0.0.0)
# Date: 2004/06/06 17:22:14

# Global parameters
[global]
    workgroup = NETZ
    server string = Samba Server
    interfaces = eth0
    passdb backend = tdbsam
    log file = /var/log/samba/log.%I
    max log size = 50
    os level = 32
    preferred master = No
    domain master = No
    default service = homes

[homes]
    comment = Home Directories
    valid users = %S
    read only = No
    browseable = No

[netlogon]
    comment = Network Logon Service
    path = /home/samba/netlogon
    guest ok = Yes
    share modes = No

[printers]
    comment = All Printers
    path = /var/spool/samba
    printable = Yes
    browseable = No

[Projekte]
    path = /data/projekte
    valid users = u1, u2, u3
    read only = No