Thanks chaps.
I sorted it in the end by booting into safe mode, loading the generic drivers XP carries for ATI then installing the AVGA drivers over the top.
Not really sure why it worked that way but it was giving me error messages about DirectX 8 needing to be installed (even though PC is running DX9). Maybe it was something to do with the Nforce being an Nvidia chipset.
Anyway all is rosy now and the cabs back up and running.