Pc’s rebooting on own.

We are having a problem a few of our pc’s rebooting on there own. They are on a network that is running Win NT. The servers are not setup to reboot any networked system. Any suggestions. P.S Could it be possible that there is a windows update that is not loaded. The systems vary, one is a 266 dell w/Win95 loaded. The other is a dell 400 w/win98 loaded. Both are XPS mini towers.

Thanks for anr info.
Brian McElmury