Well, I thought I'd solved my random crash problem, but three days later during a game of Counter-Strike it happened again. I've spent most of the weekend trying to track down the cause, and I'm leaning more towards the possibility that it's not a driver problem, but a memory access violation being caused by Steam.
However, I can't remember if I had Steam active during any of the other crashes! Either way, it's bound to happen again, so I've made a couple of adjustments to the error reporting options in XP to see if I can trap a little more information when it does. I am determined to nail the little gremlin causing this annoying problem!