Cannot logon to my server with XP pro or 98 Clients.

I just installed a Windows 2003 SBS to my network and I am
having trouble with setting up the new domain. I currently
have a Windows NT server and a 2000 server.
My NT server is the PDC and to get the 2000 server to
connect I had to disable active directory. when setting up
the new server I created a "mmcoa.local" like I was
supposed to. my orignal domain was just "mmca" I can get
the 2000 server to connect to the new domain but I cannot
get any of my XP Pro or 98 clients to connect. they all
say path not found or wrong password/logon disabled.
What am I doing wrong?
is there any way I can get the 2003 server to take over
the NT domain? I don't think it's possible but I figured I
would ask.

any help would be greatly appretiated.
Thanks