Server logs off local machines after 6pm

I use server 2003. I have 25 computers on the network, 5 network printers and 3 local printers. I also have Sage ACcpac (Pervasive) installed on the finance manger’s machine. He always complains that at around 6pm, the server logs him off and he looses a lot of work…

Can someone help what may likely be the problem.