Windows 2000 Server Slow after 7 days

Our windows 2000 server has been working without problems since may/june 2000. But in the last few weeks the server runs very slow after 7 days and gives an event error 2019 just before a warning 5781. If the server is re-booted all is fine for another 7 days. The problem is always over the week end when the server is not being used. Users cannot logon first thing monday morning, but PC’s that are logged on are still connected, but data transfer is very slow.