I need to configure samba as domain controller for windows XP clients,I followed a lot of steps but windows machine telling that "Unable to join into the doamin Username could not be found".How to fix this problem.give me an introduction about what are the parameters needed for domain controller in smb.conf and what are needed for roamin profile.Please help me in this problem.I am confused .Please help me...
Thanks .

Each user needs to have an account on the nix machine.
They also need to have samba accounts (using the smbpasswd command).

Roaming profiles are actually fairly easy.

Do it a step at a time. Once you are able to log on then work on the roaming profiles.

I highly recommend checking out:
www.samba.org
and read up on the Learn Samba : By Example section. Very helpful.

I'd give you more but my machine is away right now.

You should put your distro in your user profile. Your distro may have a wizard configuration that will do the heavy lifting for you. Also look in using samba swat. Also, your distro may include the books mentioned in the prior post with the samba package or a separate samba-doc. ( Samba 3 by Example & Samba 3 HOWTO & Reference ) An edition of Using Samba is included with samba and reachable by clicking on the bottom of the samba swat web interface.

You didn't mention which XP clients you have. You are OK if you have XP Pro clients. XP Home clients can't join a domain. Microsoft stripped it out of the Home edition.

This goes for Media Center as well.

Also, don't forget that your machines must also have accounts in Samba. Add them as users on the Linux box followed by a '$', so:
If your distribution doesn't allow user names ending in a dollar sign, you'll have to manually edit /etc/passwd (using vipw) and add it before running smbpasswd.