I just wanted to post that I'm having the same problem here at my school.
I have:
Windows 2003 Server (New Dell - less than a year old)
4x Windows XP Clients (New HP's - less than a year old)
4x Windows 2000 Clients (Older Acers - several years old)
Netgear Prosafe 10/100/1000 Switch (new, replaced entire network in July)
This problem only occurs with the XP clients, the 2K clients run fine all day without problems. The XP machines run without problems until around 5:30 or 6:00 and then slow to a crawl on the network. Opening documents in Word or Excel takes a very long time (this applies only to network documents, local documents open normally). Eventually the computer will completely lose its connection to the server and give an error message when the user attempts to open the mapped drive (name already in use error).
I've tried about everything I can think of. Through different attempts I've changed antivirus software (even disabling it at one point), checked that there are no scheduled tasks on either the server or the machine, checked for all the latest Microsoft updates for both the server, the client, and Office. I've checked the network cards and cables, disabled DHCP and set static addresses, checked DNS settings, and a variety of other things (I have a growing notebook about this problem).
The problem happens consistently around the same time each day, and a reboot of the client always solves it. It's just a pain for some of my users that work late to have to reboot or re-login every day around the same time.
What I have NOT tried yet is to have my user reboot their system earlier in the day to see if the problem still happens at that time (to see if it's a clock time issue or an amount of time logged in issue). That's today's experiment.
I'm just glad to see that I'm not the only one who's having trouble with this...