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 
hey, does the guest OS have direct access to the hardware of the host? I’m trying to install Nvidia driver and some other drivers to control brightness of the panel, I was wondering if it’s possible with KVM or maybe XEN
ah, no. sorry for the confusion. all i was talking about was having 3D (hardware acceleration) still working okay on the _host_ OS while using the XEN linux kernel. i had thought the nvidia driver wouldn’t work with it but it did.
i use KVM these days but i’m pretty sure that none of the current alternatives allow for any 3D hardware stuff on guests. i think most VM systems emulate a simple SVGA card (something cirrus i think) for graphics.
i’ve also had a brief look at using the nvidia driver to control monitor brightness (on the host). it looks like it’s possible but it’s not configured by default on my fedora system. i’ll post if i ever solve that one.