Windows 2003 network freezes

We have a network of 1 2003 server with 2003 Exchange and
20 XP prof clients. Every few days the network freezes.
After a reboot from the server users can work again. I
also have trouble with computer browsing. I stoped the
computer browser on the server. When it is started users
cannot browse the network on the werkstations. We work
with DNS. WINS is off. There is also a delay in access and
deleting files on the server. This should not be becaurse
the network works on a 1000 Mbit backbone and al
werkstations have a good 100 Mbit connection to the
server. All the CISCO switches are correct configured.