W98 & XP unable to find tree

I have 5 labs of Win98 and XP machines running latest Novell clients. We
are running Netware 6 servers. With my Win98 labs on startup I get
an "unable to find tree" at the login screen on a majority of the
machines. Of the 75+ machines in 3 labs, a handfull will login on the
first try. Most can require upwards of a dozen reboots. When this happens
they can not find the tree once in windows and using the tray icon,
either. These machines are also able connect to internet and print during
this time.

The XP machines are much more forgiving and will generally spend up to a
minute searching and will eventually log in. However, even they will
sometimes have 80% of the lab that will not log in until they have been
rebooted multiple times.

Our Network manager suggested removing IPX from our Network settings since
we are running IP only; this did not work. He has mentioned that it is
probably our Hubs in the labs, however, some of the worst problems occur
in labs with each computer having a straight drop to the IDF cabinet.

Any help would be greatly appreciated, as I have 5 teachers who are unable
to conduct lessons without the server.