I have the same problem, but I'm not sure what causes it. When I started playing in early 2006, this almost never happened. Nowadays, it happens many times a day, but only when CO is running. Nothing has changed in my hardware configuration, and the game used to work beautifully on this machine. I haven't tried reloading Windows (I use XP), and I'd rather not do that because it's a pain in the ***. I've updated the graphics drivers, even completely reinstalled the card, and it didn't help.
It can happen with only one client open, but it's rare. Usually it happens more frequently the more clients are open, so I think that it's not related to multiclient, only that more clients open increases the odds of the problem occurring.
What happens is that the computer totally locks up. No response from ctrl-alt-del or anything else. I can get the num lock light to toggle on and off a few times, but then even that stops responding and the only recourse is to cycle the power to the machine.
It happens whether CO is in front or I'm working on something else. It can also happen if CO is the only thing running. It could be some issue with the multiclient, but if that were the case I'm sure many more people would have this problem. In any case running only 2 clients isn't an option.
I think it's probably crappy coding by TQ, and we all know it wouldn't be the first time that happened. Anyone else have this problem and found a fix for it?