Your video processor may not be up to speed - are you playing on a laptop by any chance? The minimum system requirements are:
* OS: Windows XP - Vista - 7 - 8
* CPU: Pentium 4 at 2 GHz
* RAM: 2 GB
* HDD: 2 GB free disk space
* Graphics: 256 MB card
* Sound Card: DirectX 9 Compatible
* DirectX: Version 9.0c
If you're running an IGP with less than 256 MB, that may be the issue.
Thanks for your thoughts.
I'm playing on a relatively high powered PC and have a GeForce GT610 (2048MB) video card so that can't be the reason.