I am occasionally seeing hard lockups on XP: totally unresponsive to keyboard/mouse, screen freezes at time of crash, no SSH/VNC possible. Very intermittent, nothing in the logs. I never see a blue screen on any kind of error message.

This morning I logged in via VNC, logged out again, 20 minutes later physically sat at PC and it crashed around the time of VNC logout.

I tend to suspect video cards in these kind of situations but it's a modern-ish card with modern drivers (one revision back, but this has been happening for 5 revisions or more) and normally would at least see a blue screen I expect.

What would you suspect? Where can I look or what can I set up for more information? Bear in mind that this happens about once every 3 or 4 weeks, so extensive logging or intrusive monitoring isn't really an option.