Lost/Crawling Network Access.

I handle a 50 computer network in a small school.The OS’s on the client machines range from WinXP to Win2000 Professional clients.An initially minor problem is fast becoming a big one which is that most of the winXP computers cant log onto the network after a few hrs of inactivity.those that do log on, are very very slow to respond.on one of the client machines, i had to log onto the local machine(instead of the domain)b4 i noticed a substantial increase in response time which i cant explain.is the problem wt the local machines(network idle time out) or the network.I need a solution fast..