XP and 98 network computers lose default printer

I manage a mixed OS network of about 50 computers. The
server is NT and the majority of the workstations are 2000
Pro. But the computers in question are either 98 and XP.
The Win 98 systems lose their ability to print to their
default printers, and restarting corrects this, suggesting
to me that it is a network problem. The Win XP computers
do something else entirely (actually two things). [1] The
print GUI shows a blank line where the printer should be
at the top of the pull-down menu. Scrolling down to the
correct printer works, but the problem is aggravating for
students who don't know beans about computers. [2] One or
two XP computers will prompt you to select a default
printer using the Control Panel. When you look, there is
one already selected. The computer (or perhaps the
applications) does not accept the current default printer
setting. However, when this happens it involves several
applications. We are using CleanSlate and server-assigned
user privileges, and dynamic IP addressing.