Its been a couple of days without a BSOD, so the admin might as well mark this problem solved. If the BSOD pop up again in the future, I'll start a new thread at that time.
The problem (as far as I can tell) was caused by my antivirus (Kaspersky). After running every check I could find on the hardware, it all came up clean.
I was getting a seemingly random spread of crash messages until I disabled the UAC. For about a day, there were no crashes, but then they started again. This time, they all pointed to one source... kl1.sys, a Kaspersky file. After three BSODs in a row all named kl1.sys as the source, I removed Kaspersky and switched to AVG. No problems since.
I think Kaspersky and some part of the UAC were conflicting with each other, causing whatever file they were fighting for to crash... and that file would get the blame... hence the wide variety of different "causes" in the WhoCrashed reports.
Shutting down UAC let the true problem shine through.
Thanks to torrentg, hammerhead, usasma, and anthony for your help. I never would have known where to start looking without y'all!