IBM PC Restarts during boot

I have an IBM machine that will not boot.It’s running Windows 98 2nd ed. I can get as far as entering a password at the network login box, and then the machine shuts off and restarts. I managed to get it into safe mode, and have fooled around withsome drivers–video, etc., but nothing helps. I replaced the RAM, and the power supply, with no effect. What’s up?