Thanks Tom. On your suggestion that the problem lay elsewhere, I tried several other things and finally stumbled on a solution. For others who might encounter a similar problem, I have an AMD Radeon HD 5700, and updated the driver from the AMD website. Hadn’t thought this was a problem initially as I had used their autodetect to update already, but looking through the website I found there was a newer update that wasn’t covered by the automatic update. The game works perfectly with this newer update, but not with the one that any automatic update installs.
Thanks again for your help.