Domain users must Restart to Reconnect to Server

I have a domain network of 7 users running windows XP Pro and a Copier/Network printer. Everyday at different times of the day with every user at different times, they must reboot there pc to reconnect to the Domain to have access to folders on the Domain Server. I have replace all the switches, router and cables thinking it was a network issue. I have reformated the PC not reinstalling from a Ghost but Reinstall OS and Drivers. The printer is turned on in the morning and off at night. PC turned off same way, but the printer is always connected. I thought it might be the PCs still so I took my laptop that works on other networks, and after a day use, i had to reboot it to log back on to the server. There is no session, basically file share. I have turned all the group policies off, and still and issue. Im stumped.

The system is a dell 2800 server with SBS 2k3 running on it.

 

Well I tried that out and it look to work, but today a couple people had to restart there pcs. Is it there something in server?

 

Very strange. Haven't seen this before. Make sure you let us know if you figure it out?
Why reboot? Have you tried Log off and log back on?
Have you looked at the event logs on the server?