Computer Kicks itself out of the domain

[font=&quot]Ok Here is a one I have been working on for a long time.
I have two computers on my network one a 2000 and one XP.
It seems like every so often it will kick its self out of the domain.
What I mean by this is that when the user tries to login they get a
message saying it can't find the domain controller.
Now I know the domain controller is there because everyone
else is able to login and out with no problems.
To be able to get the computer to login into the domain I have to login locally and then change the domain to be a workgroup. Reboot. Login again connect it to the domain and then reboot one more time.
After all of this I am able to login into the domain with no problems

It seems to happen more often on the 2000 computer than the XP
the 2000 it seems to happen about once a week or so...maybe when they restart that computer maybe...but I am not sure if that is the case.

The XP Computer has not done this in while at least as far as I know.

does anyone know where to look to fix this problem on these two machines...as I said it is only these two and we have around 40 computers on our domain.

any help would be cool
Thanks in advance.[/font]