I had a very similar problem. Here's how I solved it:

1. Go to: control panel-->appearance and personalization-->display-->adjust resolution
2. Change the resolution to the desired ratio (in this case likely 1200x800). This should make the screen look stretched/deformed.
3. Click on advanced settings
4. Open the tab for your graphics card and click on the button to open its control panel
5. For NVIDIA (this may be different since I have a different NVIDIA card on a laptop), under display, select "change flat panel scaling"
6. Select "use NVIDIA scaling with fixed aspect ratio" and apply

This worked for me. To revert back to wide screen, you only need to do steps 1 & 2 (but you'll have to repeat all 6 steps again if you want to change it again). I find it a bit tedious to switch back and forth, but I couldn't find any other way that worked.