I tracked it down to a bad driver. Another machine has a bad network card. And another bad ram.
The list is far too long to keep going but why not look at the EVENT VIEWER for clues?
I'm running XP Pro SP3. and have several computers on my network through a wired router. Only on one of the computers am I having a problem. I can view view files on one of the peers, but when I change the file, the peer computer reboots.
Changing means, if I open a Word document for example, and make changes, then save those changes, the computer that had the original file reboots. Likewise, if I copy a file from one computer to this particular machine, if the file already exists, I get the warning (do you want to replace/overwrite), and when I confirm, the peer reboots.
Where should I be looking to correct this behavior.