It's quite possible it's an EDTV (Extended or Enchanced Definition TV), which somewhere in between SDTV and HDTV. They often meet the vertical pixel count for an HD image, but fall short in the horizontal pixel department.
You can generally add custom resolutions with your GPU driver control software. How you do that depends on what GPU your machine has.
NVIDIA
You can use this guide here: http://www.nvidia.com/object/custom_resolutions.html
- Browse to the NVIDIA Display Properties by right mouse clicking on the Windows desktop and selecting NVIDIA Display.
- Select the Change Resolution option.
AMD
With AMD you use the Catalyst Control Centre, but they don't seem to have an official guide and the only machine I have with an AMD/ATI GPU is my HTPC which I currently don't have access to. However, the method should be very similar to way you set it up with NVIDIA.
I will update this portion of the answer when I have access to the HTPC.