Using XP SP3, NIS 2011. On 8/1/2011, (1:51 PM EDT according to the dmp file), I suddenly started getting BSODs. I noticed that this only happened when the machine was idle. I'll skip everything I tried and get right to what I thought was wrong and what worked.
I found that my NIS idle time delay was set to 10 minutes. So was my screen saver delay. this may have been a coincidence, as changing the screen saver delay didn't fix the problem, but changing the NIS idle time out to 30 minutes seems to have worked.
Note that the 10 minute configuration has been in place since NIS 2009 with no problems.Perhaps there is something else that comes along every 10 minutes- I don't know.
Examining the first dump file pointed to SYMEVENT.SYS ( SYMEVENT+143d9 ). Subsequent dump files pointed to all kinds of different things, all kernel related, and I ruled them out figuring that symevent interacts with the kernel, and Windows is famous for giving wrong error messages.
Anyway, it's been about 24 hours now, and no further BSODs.