XP Pro Looses Connection to Domain

We have a win2K server acting as a domain. As workstations we have seveal win2k pros, one me, a few win98’s and two xp pro machines. The users with the xp pro machines loose the connection to the domain overnight. They leave their computers running overnight and when they come in the morning they have been disconnected from all mapped drives. A re-boot solves the problem. Why is this happening and only happening on the xp machines?