You should be able to reinstall it from the Services tab in the Network Properties. When you reinstall the service, it installs the srv.sys file from the original NT install files. So, it may not be starting because you have not re-run the latest service pack, so be sure to do this as well.
If it still won't start, it could be because the bindings weren't updated properly. You need to check to be sure that TCP/IP and WINS are properly bound to the Server service.
If this doesn't work, you will probably have to uninstall and reinstall all of the networking components - NIC, protocols, etc. To do this, first uninstall TCP/IP and then uninstall the NIC drivers. You _might_ be able to get away with just uninstalling and reinstalling TCP/IP without the NIC drivers. This usually will automatically uninstall and reinstall all of the other network services (Workstation, Browser and Server services). Once everything is reinstalled, then run the latest service pack (6a) again.
If none of this helps, you need to look at your event log and see if there are any messages appearing in the System log when you try to start the Server service. Troubleshooting those events will probably help. Hope this helps!
