Just before getting the call about the branch I was (again) cursing the fact that the Win32 installation doesn't have a graphics driver. Well, with the break-time (including talking to Soni) I realised the likely culprit: AGP bus failures due to attempts to use "advanced" (but poorly implemented) features.

Poke through the BIOS a bit and yes, it allows for disabling all sorts of AGP-related features. Disable basically all of the advanced features, reduce to the lowest allowed AGP multiplier and presto-chango, working Win32 display drivers. I'll take some time some day to track down which particular feature causes the problem and enable the rest, but for now I'm happy just not having to stare at 800x600 16-colour graphics any more.