Client connection problem

Here at the University, we have a server setup in the Chemistry
department. Students should be able to connect to that server from some
XP clients we have in one of our classrooms. What we have found though,
is that some computers can, while others cannot. I thought there migth
be a server licensing issue, but I looked at that and they have 100
licenses and the peak use is 13. We had the network people check the
network between the two rooms and they found nothing strange. The
computers can connect to other servers around campus w/o any trouble, so
I at a loss as to why this is happening, but my gut tells me there is a
server config problem. When using "Map Network Drive", the user is
presented with the auth dialog box each time he enters the password. I
tried a net use command from the prompt and received an "Access Denied"
error. By the way, they all using the same account to log in with.
Any ideas?
TIA