Built game only works properly full screen

I don't even really know how I can show you all this issue. I've never heard of it before, and couldn't find an answer with a search, so... here I am.

I built my game, windows standalone, works great when I'm in full screen mode. However, I thought it might not be a bad idea to do a little QA with it windowed. Once I hit play, the character flew across the screen, affected by gravity, so it stopped once it fell into a little ditch, and died. I have no clue what could be causing the character to float to the side all on it's own, but if you guys have any ideas, I would be eternally grateful.

Thank you.