well, i followed the instructions here and restored my old XP vm image. and 3D stuff still works. i guess maybe the two weren’t connected when it went wrong on the other computer.
of vague interest to some: first i installed the VM as an x86_64 system (yes, running 32bit XP) with two cores. it ran fine but used one of my cores flat out permanently (qemu-kvm). re-installing as an i686 system with one core sorted that out. not sure which of those two things caused/fixed it though.
hooray! right, back to the 3D programming then. just as soon as i’ve eaten something 🙂