I tried the following command on RedHat Linux Enterprise Server 4 to add a machine account so that I can access the Samba domain on the server from the XP PC.
useradd -d /dev/null -g 100 -m xp_machine_name$

The above command works on RedHAt Linux ES3, but on ES4 it is giving the foll error......
useraddd: invalid user name 'xp_machine_name$'

Finally I had to used the machine account created on RedHAt Linux Enterprise Server 3 by copying the /etc/passwd, group, shadow, gshadow & the /etc/samba/smbpasswd onto the ES4 server & joining the XP PC to the Samba Domain running on ES4.

I haven't done this for a while but here's my notes from the ibm site....

# /usr/sbin/useradd -g machines -d /dev/null -c "machine id" -s /bin/false machine_name$

# passwd -l machine_name$
Changing password for user machine_name$
Locking password for user machine_name$

The first command creates the user machine_name (don't forget the dollar-sign; it's required and identifies the entry as a trust account)), as a member of the group machines (-g), with no home directory (-d /dev/null), a descriptive entry (-c; for example, "Tom's Notebook"), and no shell access (-s /bin/false). The second command creates a "secret" for the machine to authenticate against.

With the UNIX account created, we can now add the machine to /etc/samba/smbpasswd as shown below:

# smbpasswd -a -m machine_name
Added user machine_name$