Vista - Vista machines automaticlly disconnecting from domain

I've had 4 computers on our network in the last 2 weeks disconnect themselves
from the domain. Users don't have access to be able to do this themselves. It
has only been windows vista pc's so far. I have done some googling online but
been able to find nothing about this as I thought maybe it was a virus. We
are running trend micro office scan v10 which is running fine on the 4
machines. The first machine that this happened on I just simply readded it to
the domain and it all worked fine again for a few days then was disconnected
again, then again a few more days later so I reinstalled vista on it as I
didn't have the time to look into it but now that it seems to be happening on
more machines I need to find out why.

Only thing I can find out is that the user is working fine, they turn the pc
off for the night and then come into work the next morning to find the they
can't login and I find that I can only login with the local administrator
account because it is no longer part of the domain

Any help or ideas on this strange problem would be welcome
thanks