Win ’98 ICS client gets screen of death

Two ’98 SE systems, client 512 MB RAM, host 128 MB. Internet access is via cable modem. My boss tells me he gets a blue screen sometimes when he is surfing the internet. He is not able to tell me specifics, such as whether it happens when both the host and the client are being used, or what the error message says, etc. I have been there several times (I applied patches to OS, upgraded IE to 6, removed viruses and malaware, installed Zone Alarm on the host) but have not been able to reproduce the problem. I also noticed that the host PC was set to turn off the hard disk after 30 minutes and changed that, but he tells me it is still happening. Any suggestions?