I constantly restarting ibm server

I support a lan of 35 computers which depend on 2 servers one as mail server and file server ,
the file server keeps on restarting after every 15 or 30 minutes , what could be the problem , I have done a virus scan , update the os that is win2000 server, scan the work stations but the problem is still persistent.The server is ibm e-series 200 please help