I'm using Workstation 6.0.0 under Windows XP SP2, and my guest is Windows NT 4.0 Server SP1.
The problem is: within 10-15 minutes with the VM powered on, my host system suddenly restarts even with auto-restart off and there's nothing recorded on the event log, or "the system recovered from a fatal error" message on reboot.
Is it something related to VMware?
VMware Workstation 6.0.0 is a little old. Try upgrading to either 6.0.5 or 6.5 -- available here: http://www.vmware.com/download/ws .
After upgrading VMware Workstation make sure to install the latest VMware Tools in the Guest.
On the host, also make sure to configure your AntiVirus to exclude the folder holding your Virtual Machine files.
Also, configure your Guest's Virtual CD-ROM to start disconnected.
NT 4 guests require SP6/6a.