Sudden Restarts

What I would like to know is why a system would suddenly freeze, then within a few seconds, restart by itself. This occurrs when running NetMeeting, ICUII, sometimes Outlook Express, and any video program. The system in question is a PIII 600, with 256 mb RAM, VooDoo3 16mb Vid card(AGP) running Win98 S.E. Any information would be helpful. Also when using a web cam, it will stop and restart. Help!!

Gerry Wolf