I use 1920x1080 resolution by default, and have been for over a year with this computer. Once I started up this game, it looked fine, but when I exited everything else that used 1920x1080 resolution became permanently blurry. If I switch to any other resolution it's fine, but my default resolution is now unusable. Other games (themselves set to 1920x1080) are similarly blurred. I'm using Windows 7. I'm sure this is just some little setting the game decided to change for me, but am at a loss as to what exactly.

Maybe color depth ?

Check if the game switched it to 16 bit. If it did change it back to 32 bit.