Why server(win2k) have to restart repeatedly

hello sir,

let me explain the problem here..
there are workstations comprises of win98, win2000 and 1 server-win2K...
1 server downstairs while workstations are both downstairs and upstairs..

staffs can access internet and see server...can print...
however..they cannot access internet, cannot see server and cannot print ..
it happens at interval times...then restart server..then it is
ok.......within a few hours later, problems come back again....then restart
server..ok...it happens often.
so how to resolve problem by restarting server?
cannot expect a person to restart server constantly?

waiting for your reply soon
thanks

regards
ellen