Question : domain is not available

I have a pretty regular problem with my network telling me that: "The system cannot log you on because the domain "mydomain" is not available."  I know that the domain is available as I have several users logged on with thier desktops and a few with laptops.  It only does this with laptops.  I do not understand why it happens.  Sometimes it seems to work fine and other times it doesn't work at all.  Why is this?  It happens with several of my older laptops.  The newer laptops that are online regularly dont have this problem.  It is only the older spare ones.  When I pull one out of storage to fire up I cannot get it to connect for some reason.  Any ideas?  We are a very basic network using a windows 2000 server and all XP clients.

Answer : domain is not available

Logon to the laptop as a local user first, connect to network, logoff the computer, and log back in as a domain user.