Aha kinda weird.
Suggestion 1:
Try setting the other monitor as the primary screen.
Solution 2:
Make sure the monitors are not mirroring each other and
Just change the resolution
I highly recommend you change to 1920 x 1080 and adjust overscan/underscan until the image fits your screen.
Solution 3:
Read this:
http://answers.microsoft.com/en-us/windows/forum/window...
If you have AMD Graphics card, Open AMD Catalyst Control Center(if you do not have it you should install it from the AMD website.) and select My Digital Flat Panels and then Scaling Options. You may have to go all the way to 0% on the Overscan side.
http://imageshack.com/a/img268/7339/zvsr.png
Do the similar thing if you have Nvidia. And also manage that F4 button on the keyboard, or whatever key that has Squarish monitor icon on it.
EDIT: Don't forget to change Games settings, thru the launcher or whatever. It must be games' settings, change resolution to what I recommended