I've run into this when my graphics drivers try to force AA or AF filtering. Check to see in your graphics software (Radeon or GeForce) if anything is being forced, and see if setting it to "Application Preference" helps any. - I run into it on other games as well, so I think it's a Windows thing. - Hopefully some of that helps. You can also try Lightshot for screenshots. I hear it works fairly well on Win8.
-Cheers