Client Game turning into windowed mode instead of Full screen

Recently I played a game where, during the loading screen, the client went into a sort of windowed mode (I dont know how else to describe it) where it was no longer full screen, thus half of my screen was below the line of my task bar. I spent the first 10 minutes of the game trying to fix it due to the inability to operate the bottom half of my screen (It was impossible to use the mouse to move the view around the map). After that I did some research on it (while the game was still going) and I found a post with the same issue recommending a change in the resolution. This fixed the problem in as much as I could resume the game and have full use of the view, but I still had the issue of the game being in window format and I ran into an issue of clicking on the task bar (window's button in particular) and messing my game up, often in important team fights. I have not played another game yet as I dont want the same issue to take place.