computer restarts by itself

I have an old Compaq desktop running Windows 98se. When I go to shut it down by using the Start button, it gives me options of restarting in MS Dos, restarting, or shutting down. Just recently it has decided to restart even when the "shut down" box is checked. Today I thought it had behaved itself, but a full 5 minutes later it suddenly restarted.
I've done a search of this site, but cannot do as recommended to others.......my computer does not have an "advanced" tab in "system".