Maintaining Networked Resources

I maintain about 40 computers on the network at my office. Currently one of the XP Pro machines (computer A) has been having trouble maintaining network connections. After a reboot, the machine (computer A) will connect to any computer on the network and any computer on the network can connect to it. After a few hours there is a select handful that can connect to it (computer A), and that it can connect to, when the user restarts the problem is repaired again for a small amount of time. The user has not installed any updates Windows or third-party, and has not changed any system settings.