The error is a STOP 0xD4, "driver unloaded without canceling pending operations", and appears to be caused by av5flt.sys. Google says that this file part of the Panda anti-virus program, which I know is installed on that machine.
It seemed like the obvious first step was to get into safe mode and uninstall Panda or prevent it from loading until I could figure out what was wrong. The problem is that the same thing happens even in safe mode -- when I would assume av5flt.sys should not even be getting loaded in the first place.
I was hoping someone here could give me advice on how to gain some control over what is loaded so that I can at least get into safe mode. I could swear that I'd seen an interface before where Windows tells you everything it wants to load during system boot, and gives you the option to allow or veto it, but nothing on the advanced boot menu does that.
My theory is that this is either Panda being too clever for its own good by forcing itself to be loaded even in safe mode, or else it's some kind of malware.
Another symptom that I didn't expect is that when I tried to boot safe mode with command prompt, I got the graphical shell (followed by the BSOD) instead of the command prompt.
Anyway, a lot of weird stuff seems to be going on, and my image as the IT guru of the family is beginning to tarnish, so if anyone sees anything familiar about this problem or has any advice, please feel free to chime in. Thanks.